Contents lists available at ScienceDirect Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr Enhanced grid frequency support by means of HVDC-based load control ☆ Marius Langwasser ⁎ ,a , Giovanni De Carne a , Marco Liserre a , Matthias Biskoping b a Chair of Power Electronics, Kiel University, Kiel, Germany b ABB Research Center Ladenburg, ABB AG, Ladenburg, Germany ARTICLE INFO Keywords: Frequency support Load sensitivity Multi-terminal hvdc Primary frequency regulation Voltage-dependent loads ABSTRACT The two major frequency incidents in Continental Europe and UK call for faster frequency regulation. Hvdc systems can dynamically adjust the power output to support the frequency regulation by means of droop characteristics. Adding derivative terms in the power controller enables inertia emulation that improves the system initial response. As drawback, due to the fast power transfer variation, the HVDC-connected areas suffer from temporary power imbalance and hence frequency deviation. To overcome this limitation, HVDC-based load controllers can be implemented at HVDC terminals to shape the consumption of nearby voltage-sensitive loads by means of controlled voltage variations, with the goal to minimize the frequency deviation in the supporting areas. In multi-terminal systems, the frequency support can be optimally shared among terminals depending on the load sensitivity. This minimizes the voltage deviation in supporting areas and limits the hvdc reactive power, while providing the same frequency damping in the faulty area. 1. Introduction Nowadays, the electric grid is challenged by the continuous re- placement of conventional generators with renewables ones, which, due to their power electronics grid interface, do not provide any rota- tional inertia to the grid. The result are faster and larger frequency transients, as it can be observed in the two major incidents in 2019 in Continental Europe [1] and UK [2]. As practical example, Ulbig et al. [3] has found that the aggregated inertia in the German grid is about 6 s, if conventional generation dominates, and drops to significantly lower levels of 3–4 s in times of high wind and PV production. Thus, in case of large grid contingencies (e.g. grid fault or generator/load shut- down), an increased frequency deviation from the nominal value is observed, e.g. from original 400 mHz to more than 550 mHz. Con- ventionally, synchronous generators stabilize the frequency, which, in the first seconds after the frequency deviation, adapt their power output proportionally with the observed frequency change. The same fre- quency support (FS) can be provided by hvdc systems, but with higher control dynamics. Hvdc systems with primary frequency regulation adapt their active power reference rapidly following the frequency disturbance. This strategy is widely employed in point-to-point [4] and multi-terminal hvdc (mtdc) systems [5–8]. However, methods such as the one in Chaudhuri et al. [5], have two drawbacks. First, the fre- quency/power droop controller only increases the faulty system’s damping and does not contribute to the system inertia. Second, due to the fast variation in the power transfer, the supporting grids will ex- perience temporary power imbalance and thus large frequency devia- tion of up to 500 mHz. As proposed in Rakhshani et al. [9], Zhu et al. [10], derivative terms in the hvdc power controller virtually add inertia to the faulty system. To overcome the second limitation, the hvdc terminals can be equipped with load control [11,12], which makes use of the voltage-dependent power characteristic of nearby loads (e.g. industrial aluminum plants or HV substations [13]), shaping their power output with controlled grid voltage variation in the first seconds after frequency disturbance. This concept has been already applied in SVC [14], synchronous condensers or smart transformers [15,16]. Mtdc systems can enhance the frequency controllability, since they inter- connect multiple ac areas and allow for improved power flow control. In this work, an enhanced FS with mtdc systems controlling voltage dependent loads is proposed. An optimized distribution of the FS is implemented respecting the estimated load sensitivity in each con- nected area. This implies that terminals connected to loads with the highest sensitivity provide the highest share of FS. As consequence, the control margin for both upward and downward FS is enlarged, since the https://doi.org/10.1016/j.epsr.2020.106552 Received 4 October 2019; Received in revised form 17 April 2020; Accepted 20 July 2020 ☆ The authors gratefully acknowledge funding by the German Federal Ministry of Education and Research (BMBF) within the Kopernikus Project ENSURE “New ENergy grid StructURes for the German Energiewende” (03SFK1I0 and 03SFK1I0-2). ⁎ Corresponding author. E-mail addresses: mlan@tf.uni-kiel.de (M. Langwasser), gdc@tf.uni-kiel.de (G. De Carne), ml@tf.uni-kiel.de (M. Liserre), matthias.biskoping@de.abb.com (M. Biskoping). Electric Power Systems Research 189 (2020) 106552 Available online 29 July 2020 0378-7796/ © 2020 Elsevier B.V. All rights reserved. T