Comparison of existing PV models and possible integration under EU grid specifications

Abstract: This master thesis investigates the capabilities of a generic grid-connected photovoltaic (PV) model that was developed by DIgSILENT and is part of the library of the new version of PowerFactory v.14.1. The model has a nominal rated peak power of 0.5 MVA and a designed power factor cosφ=0.95. A static generator component, which includes the PV array, the DC bus with the capacitor, the inverter and the control frame, is used to model the PV system. The PV array is considered to operate at the MPP and the generator with cosφ=1. The thesis begins with a short review of the current status of the PV sector, focusing mostly on the types of PV systems and the necessary components that are used in grid-connected systems. Since the PV inverter is the key component, special reference is made to the different technologies applied and to the multifaceted role that inverters should play nowadays supporting the grid’s stability. Technical restrictions and requirements arepresented highlighting primarily the German Grid Code for the MV network, which is the benchmark for the analysis of the role and behaviour of the PV model in question. Germany is regarded a very good example to base the study on due to its leading position and experience in the renewable area and its thorough grid specifications. The main part of the report includes a detailed description of the structure of the generic model, presenting the operating procedure of its components as well as model assumptions and simplifications. Various simulations in variable solar irradiation, frequency and voltage conditions are performed in order to conclude in its capabilities. The static voltage support is investigated under cloud effect situation where the changes in active power output of the PV array can influence the voltage stability of the grid at the PCC. The active power control is examined by forcing the grid frequency to deviate beyond specified limits and observing the active power output results. At last, the dynamic voltage support capability (LVRT) is examined by simulating four different short circuit events creating four different voltage dips. The ability of the PV inverter to stay connected and to provide reactive current when necessary is seen. The external grid component is designed to represent a strong grid. The results showed that the model is capable for active power reduction and LVRT behaviour. However, the absence of reactive power control makes it inapplicable for static voltage support. Thus, a PI controller is implemented in order to supply constant reactive power in steady state operation and support the grid stability. At last two different interconnections were built using a slightly modified version of the same generic model with a rated power 1 MVA. The control scheme remained the same. Both configurations were examined statically and dynamically and their results were compared. Small differences were found in terms of reactive power  onsumption/injection at the PCC.