“20th PSCC 2018 papers submission and review platform

Full Program »

Impact of distributed generation’s fault ride through strategies on transient stability in the transmission grid

I. INTRODUCTION
Because of the worldwide conversion of the power supply structures due to national and international climate protection programs, renewable energy sources (RES) increasingly substitute the power infeed from large conventional power plants. Large offshore-wind parks are connected via HVDC-links or directly to the high or extra-high voltage level. However, the majority of all RES is installed at the low and medium voltage levels due to their low power ratings. In Germany, the low voltage (LV – 0.4 kV) and medium voltage (MV – 10, 20 or 35 kV) levels comprise approximately 75 % of the overall RES capacity.
Increasing transmission distances, the weather dependency and the resulting intermittent nature of photovoltaic units and wind turbines enhance the volatility of the system, temporarily stressing the transmission grid. In addition, large conventional power plants providing most ancillary services are shut down. These structural changes enforce a temporary operation close to stability margins. At the same time, increasing penetration levels of distributed generation units align with an increasing impact of the distribution grids on power system stability. Therefore, the questions arises, how the fault ride through (FRT) behavior of RES, installed at LV and MV level, impacts power system stability within the transmission grid.
II. METHODOLOGY
Depending on their voltage level, different grid codes define the compulsory FRT capabilities of RES. In Germany, specific requirements regarding the FRT behavior for units within the extra high voltage (EHV – 220 or 380 kV), high voltage (HV - 110 kV), MV and LV level exist. This paper presents an approach for generically enhancing a given transmission grid model by applying an aggregated distribution grid model for the representation of the distribution grid level. The dynamic parameters of each present technology are derived from the compulsory FRT behavior of the HV, MV and LV level.
In a first step, the algorithm defines equivalent generators for each technology and voltage level. It adapts the nominal power as well as active and reactive power injections according to typical distribution coefficients depending on voltage level and technology. The electrical dimensioning of the LV/MV and MV/HV transformers is premised on the installed generation capacity and load at each voltage level. A coupling reactance connects each equivalent generator to the LV side of the transformer. The maximum coupling reactance of each equivalent generator is derived based on the maximum power infeed, defined overhead line or cable parameters, thermal limits and considering the maximum admissible voltage drop with respect to the voltage band. Thus, the coupling reactance of each equivalent generator can be varied between the minimum (neglecting the coupling reactance) and the maximum electrical distance, in order to represent different scenarios evaluating the impact on the FRT behavior.
Next, an optimal power flow determines steady state voltages and power flows. In order to maintain a voltage level of approximately 1 pu at the low voltage side of the MV and LV transformers, the tap ratio of these transformers is adapted. The algorithm adjusts loads at each voltage level to incorporate losses within the distribution grid to match the initial residual power flow at the EHV/HV transformer. The process of adapting the loads affects the resulting grid losses and thus must be repeated iteratively until actual and initial power flow match exactly.
III. EXPECTED RESULTS
In the context of the full paper, the presented approach is applied to a realistic network model based on the IEEE 118 bus network, which has been expanded by dynamic models and parameters in previous work. The impact of an enhanced modeling of the distribution grid levels is discussed with regard to various scenarios. By varying the parametrization of the equivalent generators of each voltage level, different admissible FRT strategies according to the grid code are simulated. Based on these findings the impact of specific FRT strategies on system stability is evaluated. Finally, this paper gives recommendations for improving system stability by adapting FRT requirements for RES in the distribution grids.

III. ACKNOWLEDGEMENT
This work has been performed within the publicly funded research project “SwarmGrid”. The project is embedded in the interagency research initiative “Future-Proof Power Grids” and supported by the German Federal Ministry of Education and Research (BMBF) under grant number 03EK3568A.

Author(s):

Janek Massmann    
Institute for High Voltage Technology, RWTH Aachen University
Germany

Philipp Erlinghagen    
Institute for High Voltage Technology, RWTH Aachen University
Germany

Armin Schnettler    
Institute for High Voltage Technology, RWTH Aachen University
Germany

 

Powered by OpenConf®
Copyright ©2002-2014 Zakon Group LLC