ABSTRACT:
This paper presents a grid-forming control (GFC) scheme for two-stage photovoltaic (PV) systems that maintains power reserves by operating below the maximum power point (MPP). The PV plant in GFC mode behaves like a voltage source that supports the grid during disturbances in full or limited grid-forming mode as per the reserve availability. This is a model-free method that avoids the estimation of MPP power in real-time commonly done in the literature, which makes it simpler and more reliable. The proposed control also features an enhanced current limitation scheme that guarantees containment of the current overshoots during faults, which is not trivial in voltage-sourced GFC inverters. A thorough investigation is done, exploring various generation mixtures of synchronous machines (SM), GFC and grid-following (GFL) inverters, and all common disturbances, e.g., load change, faults and irradiance transients. The results show very favorable dynamic performance by the GFC inverters, far superior to GFL inverters and directly comparable to SMs. It is found that replacing SMs with GFC inverters may improve the frequency profile and terminal voltage during disturbances, despite losing out in the mechanical inertia and the strict inverter overcurrent limits.
KEYWORDS:
1. Ancillary services
2. Current limitation
3. Grid forming control
4. Grid support, inverter
5. Maximum power point
6. Power reserves
7. Renewables integration
8. Solar photovoltaic (PV)
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig. 1. The overall GFC scheme for a two-stage grid-connected PV plant,
(a) PV system topology, (b) DC-DC
converter control and outer inverter control loop, (c) inner inverter
control loops with current limitation scheme.
EXPECTED SIMULATION RESULTS:
Fig.2.
Terminal voltage and reactive power injection in the SM GFC case subject to a
three-phase fault
Fig.
3. dq components and magnitude of the inverter current, and magnitude of
the VSC reference voltage in the SM GFC case subject to a three-phase fault.
Fig. 4. Frequency and power injection in the SM GFC case subject to irradiance change.
Fig.
5. Performance of the proposed control strategy in the SM GFC case subject to
irradiance change.
Fig. 6. Frequency and power injection in All GFC case subject to a load
change.
Fig.
7. Voltages and reactive power injections in All GFC case subject to a
three-phase fault.
Fig.
8. Frequency and power injections in SM GFL case subject to a load change.
CONCLUSION:
This paper introduces a new GFC scheme for PV systems that do not employ real-time estimation of the MPP and make optimal use of the limited power reserves. By operating in full or limited grid-forming mode, the PV plant preserves its voltage source nature and manages to assist the grid during disturbances similarly or even better than synchronous machines. The modified current saturation scheme performs smoothly, without any need for fault detection or control switching.
Replacing SMs with PV GFC results in improved
frequency profile during load disturbances due to faster response from the PV
plant, and comparable terminal voltage profiles during faults despite the
strict inverter overcurrent limits. However, the PV GFC introduces another
source of disturbances to the power system resulting from irradiance transients
during cloud movement.
Inverters in GFL mode with ancillary services can support the grid during disturbances, but the contribution becomes limited as the system strength decreases. The GFC mode of inverter operation is the way forward for the renewables-rich and inverter-dominated power systems of the future.
Future work involves a complete investigation of the
dynamic interactions between GFC and GFL inverters and the rest of the power
system at various sizes and generation mixtures. Similarly, a methodology to
determine the appropriate ratio of GFC and GFL resources would be very useful
in converter-dominated power systems. Furthermore, the proposed method is
designed for uniform illumination, which is the common assumption for
utility-scale PV systems; an extension of the method to partial shading would
improve its credibility and reliability at all possible conditions.
REFERENCES:
[1] F. Milano, F. Dörfler, G. Hug, D. J. Hill, and G. Verbič, "Foundations and challenges of low-inertia systems (Invited Paper)," Power Syst. Comp. Conf. (PSCC), Dublin, Ireland, 2018.
[2] C. Loutan, P. Klauer, S. Chowdhury, S. Hall, M. Morjaria, V. Chadliev, N. Milam, C. Milan, and V. Gevorgian, “Demonstration of essential reliability services by a 300-MW solar photovoltaic power plant,” National Renewable Energy Lab. (NREL), Golden, CO, United States, Rep. NREL/TP-5D00-67799, 2017.
[3] ENTSO-E, “Need for synthetic inertia (SI) for frequency regulation: ENTSO-E guidance document for national implementation for network codes on grid connection,” ENTSO-E, Brussels, Belgium, Tech. Guideline, Jan. 2018.
[4] J. C. Hernandez, P. G. Bueno, and F. Sanchez-Sutil, “Enhanced utility-scale photovoltaic units with frequency support functions and dynamic grid support for transmission systems,” IET Ren. Power Gen., vol. 11, no. 3, pp. 361-372, Jan. 2017.
[5] C. Guo, S. Yang, W. Liu, C. Zhao, and J. Hu, "Small-signal stability enhancement approach for VSC-HVDC system under weak AC grid conditions based on single-input single-output transfer function model," IEEE Trans. Power Del., to be published. DOI: 10.1109/TPWRD.2020.3006485.
