Available online at www.sciencedirect.

com
ScienceDirect

Energy Reports 9 (2023) 478–483

www.elsevier.com/locate/egyr

7th International Conference on Renewable Energy and Conservation, ICREC 2022 November
18–20, 2022, Paris, France

Large-scale electrolyzer plant integration to the electrical grid:

Preliminary investigation of VSC-based solutions
Rasmus Jakobsen, Chunjun Huang, Tonny W. Rasmussen, Shi You ∗
Department of Wind and Energy Systems, Technical University of Denmark, Elekctrovej 325, 2800 Kongens Lyngby, Denmark
Received 5 September 2023; accepted 6 September 2023
Available online xxxx

Abstract
This paper presents grid-code considerations that must be addressed when connecting large electrolyzer plants to the
electrical grid. A model for an electrolyzer module based on a grid-connected three-phase H-bridge converter is presented,
together with a scaling method, for investigating relevant aspects. A low-voltage-ride-through strategy is implemented into the
converter and the performance of the strategy is tested against running the system with no low-voltage-ride-through strategy.
Finally, the paper will conclude on the ability of the electrolyzer plant to provide reactive power support and the considerations
that must be made if this support is desired.
© 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the scientific committee of the 7th International Conference on Renewable Energy and Conservation, ICREC, 2022.

Keywords: Electrolyzer; Hydrogen; Grid implementation; Converter; Three-phase H-bridge; Reactive power; Current limiter

1. Introduction
The implementation of renewable energy sources is happening across the world and the amount of renewable
energy is steadily increasing. A downside of renewable sources is that the available energy is not always produced
in the instant which it is needed. When there is too much wind or sun compared to the electrical demand, the
wind turbines and solar panels are curtailed to avoid overproduction. All curtailments of renewable energy sources
represent missed opportunities of utilizing green energy. To avoid curtailment of renewable energy sources the power
can be used to perform electrolysis, where the electrical energy is stored in hydrogen. The hydrogen can then be
used in industry or as fuel for vehicles.
The electrical behavior of an electrolyzer cell is a well-known phenomenon and is studied in multiple papers. The
most common types of electrolyzers are the Alkaline Electrolyzer (AEL), Polymer Electrolyte Membrane (PEM),
and Solid-Oxide Electrolysis (SOE). AEL has been commercially available for years and is a mature technology
but are originally not designed to live on intermittent power sources. PEM is an alternative that has a higher current
∗ Corresponding author.
E-mail address: shyo@dtu.dk (S. You).

https://doi.org/10.1016/j.egyr.2023.09.061
2352-4847/© 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:
//creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the scientific committee of the 7th International Conference on Renewable Energy and Conservation,
ICREC, 2022.
R. Jakobsen, C. Huang, T.W. Rasmussen et al. Energy Reports 9 (2023) 478–483

density but suffers from increased cost [1]. SOE operates at higher temperatures which decreases the electrical
requirements. SOE shows large potential and could be the future electrolyzer cell type [2].
The electrolyzers which are already implemented are of a smaller size and usually have a capacity of a few
MWs or kWs [3]. Implementing electrolyzers on a larger scale requires additional considerations on the converter
and electrolyzer setup. This is mainly due to the high current and low voltage that are required by the electrolyzers.
Many modern converters struggle with providing high currents as the main components, the IGBTs and the diodes,
have current limits which makes the interface to the electrolyzer troublesome. Different topologies are discussed in
[4] describing the pros and cons of using different topologies. Four solutions for the converter setup are investigated
and discussed. Three of the solutions use thyristors as the main current carrying components. Although the thyristors
can carry more current than the IGBTs, additional grid supporting components are required to comply with various
reactive power demands in the point of connection (PoC). The fourth solution uses IGBT which eliminates the need
for additional reaction power support. The drawback is that many IGBTs are required to construct the converter
and that the current limit of the IGBTs often is low compared to the thyristors.
In this work, the electrolyzers are modeled using the approach from [5] and [6] to describe the behavior of
the electrolyzer. The selected converter is a voltage-source converter that can both control the voltage applied to
the electrolyzers and the reactive power production/consumption at the point of connection. All simulations are
performed in MATLAB/Simulink.

2. System model
The proposed system for the performance of the simulations is an infinite single-bus system. Using a single-bus
system the direct response of the electrolyzer and converter are observed and not influenced by other factors in the
system. The model of the electrolyzer plant is constructed from two models: the electrolyzer and the voltage source
converter (VSC). The electrolyzer is modeled using [5] for the electrolyzer current calculation part and [6] for the
remaining parts. The electrolyzer cells can be coupled to have a nominal operation point at a desired level. This
is done by coupling the electrolyzer cells in series to increase the nominal voltage and in parallel to increase the
nominal current. The nominal voltage and current should be chosen such that the limits of the converter hardware
are not violated, or vice versa.
The circuit of the proposed electrolyzer is seen in Fig. 1. The electrolyzer is divided into different sections.
A diode to avoid reverse operation, the calculations of the Ohmic part, the Ohmic part itself, the double-layered
capacitance, the calculation of the current, and the reversible voltage.

Fig. 1. Circuit of the proposed electrolyzer module, with DC power as input.

The circuit in Fig. 1. can be expanded to contain multiple electrolyzer cells in series and parallel. To couple
multiple electrolyzers in series and create an electrolyzer stack the Ohmic resistance must be multiplied by the
number of cells in series, the capacitance divided by the number of cells in series, and the reversible voltage
479
R. Jakobsen, C. Huang, T.W. Rasmussen et al. Energy Reports 9 (2023) 478–483

multiplied by the number of cells in series. The aim of adding additional series cells is to increase the voltage
without increasing the current. Adding multiple electrolyzer stacks in parallel is done to achieve a higher current
but keep the same voltage level. To achieve this the Ohmic resistance must be divided by the number of parallel
stacks and the capacitance multiplied by the number of stacks. The reversible voltage is kept the same. The nominal
voltage of the electrolyzer module is selected to be 1000 V and the current 1800 A. Running the electrolyzers at
1.8 V each at nominal voltage results in 555 cells in series and 19 stacks in parallel. This results in 1.8 MW per
electrolyzer. The V-I curve of the proposed electrolyzer module is found in Fig. 2.

Fig. 2. Behavior of proposed electrolyzer module.

2.1. Voltage-source converter

To supply the electrolyzer with DC a VSC is used to rectify the AC coming from the grid. As the nominal power
of each electrolyzer is 1.8 MW a converter with multiple output connections is needed. The H-bridge converter has
multiple outputs and can be connected to multiple electrolyzer modules. The size of the electrolyzer plant can be
selected by adding more bridges to the converter. In Fig. 3 the H-bridge converter is seen with three outputs at
each phase which is an electrolyzer connected to each output. More outputs from the converter could be inserted to
increase the power level of the entire plant. The hardware selected for the converter is an L-filter, a DC capacitor,
an IGBT, and a diode. The values of the components are seen in Table 1.

Table 1. Component values for the VSC hardware.

IGBT Diode Filter DC capacitor
Variable name Rd,I G BT [m] Ud,I G BT [V] Rd,diode [m] Ud,I G BT [V] R f [] L f [µH] Rcap [] Ccap [mF]
Component value 0.333 1 0.15 1 0.01 200 0 308.86

The control method used in the VSC is a dq-controller controlling the DC voltage and the reactive current,
providing both the ability to control the power consumption of the electrolyzer and the reactive power production
at the PoC. Additionally, a reactive power strategy and current limiter is implemented. The reactive power strategy
follows that of the Danish grid code for wind generators (Energinet) and the current limiter limits the Id reference
current such that the total current will never exceed the limits of the converter. The above-mentioned strategy and
limiter can be seen in Fig. 4.

3. Test system
The converter and electrolyzer are connected to an infinite bus to show the responses and performance of the
plant. A strong grid with a resistance of 1 m and 1 µH is used during the simulations.
Two simulations will be performed. The first simulation (case 1) will only consider a voltage drop in the grid and
the second (case 2) will include the current limiter and reactive power strategy. The two simulations are compared
based on their ability to stay within current limits. The grid voltage is dropped to 0.75 p.u. and kept there for 1.25 s,
which corresponds to the Danish grid code [7]. When reactive power compensation is needed the Danish grid code
480
R. Jakobsen, C. Huang, T.W. Rasmussen et al. Energy Reports 9 (2023) 478–483

Fig. 3. (a) Reactive power strategy. Saturation at 9500 A to always have room for some active power; (b) Id current limiter.

Fig. 4. Proposed system with the electrolyzer modules connected to the grid connecting converter through a filter.

for wind generators is followed where 50% reactive current of the nominal current is required at 0.75 p.u. voltage.
The system will have 1 s at start-up to reach a steady-state operation, at 1 s the grid voltage drops to 0.75 p.u., and
at 2.25 s the voltage is restored to 1 p.u. The peak nominal current of the system was selected to be 10,000 A.

3.1. System limitations and assumptions

The electrolyzer is assumed to maintain constant temperature and pressure throughout the simulations. Changes
in these will affect the operation of the electrolyzer. Given the small timeframe of the simulation, it is assumed that
temperature and pressure can be kept constant.
The control for the converter has the limitations of only working for a balanced three-phase operation. As the
simulation will consist of balanced faults this will not affect the simulations. If one wishes to assess the unbalanced
performance, a control scheme able to handle this must be implemented.

3.2. Results and analysis

The simulation with no reactive power compensation and current limitation (case 1) is shown in Fig. 5. When
the voltage drop occurs the current increases to compensate for the lower voltage. It is observed that the current is
481
R. Jakobsen, C. Huang, T.W. Rasmussen et al. Energy Reports 9 (2023) 478–483

much higher during the voltage drop than during normal operation. The active power is slightly higher during the
voltage drop due to increased currents. The reactive power shows smaller oscillations during the voltage drop than
during normal operation.

Fig. 5. Case 1: (a) DC voltage of the electrolyzers on phase A converter leg; (b) Grid current; (c) Active power; (d) Reactive power.

Implementation of the current limiter (case 2) is presented in Fig. 6. A larger oscillation in the DC voltage across
the electrolyzers is observed. The grid peak current is now limited to 10,000 A as were the limit selected for the
converter. In addition, it is observed that the DC voltage, hence active power, is downregulated to make room for
the reactive power production.

Fig. 6. Case 2: (a) DC voltage of the electrolyzers on phase A converter leg; (b) Grid current; (c) Active power; (d) Reactive power.

4. Conclusions
This paper presented a method to model and implement large-scale electrolyzers into the electrical grid. The
method of sizing an electrolyzer module was presented and the grid connected converter was chosen to comply
with the requirements of the selected electrolyzer. During the simulations, a low-voltage-ride-through scenario was
performed to assess the ability of each strategy to provide reactive power and keep a safe operation of the electrolyzer
and converter. With the reactive power support, the converter can deliver reactive power to the grid, but at the cost
of increased currents. Implementing a current limiter into the system causing the Id reference to lower the active
power and make room for reactive power support to the grid.
482
R. Jakobsen, C. Huang, T.W. Rasmussen et al. Energy Reports 9 (2023) 478–483

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could
have appeared to influence the work reported in this paper.

Data availability
Data will be made available on request

Acknowledgments
This work is supported by “GreenHyScale”, Denmark (No. 101036935) and “IECC”, Denmark (No. 8087-
00019B). The project GreenHyScale has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 101036935. The project IECC is granted by the Danish Innovation
Funding, Denmark. In addition, the Ph.D. student, Chunjun Huang, is jointly supported by the China Scholarship
Council and Technical University of Denmark.

References
[1] Guo Y, Li G, Zhou J, Liu Y. Comparison between hydrogen production by alkaline water electrolysis and hydrogen production by
PEM electrolysis. Earth Environ Sci 2019;371. http://dx.doi.org/10.1088/1755-1315/371/4/042022.
[2] Kamlungsua K, Su PC, Chan SH. Hydrogen generation using solid oxide electrolysis cells. Fuel Cells 2020;64:4–649. http://dx.doi.org/
10.1002/fuce.202070602.
[3] Kiaee M, Cruden A, Chladek P, Infield D. Demonstration of the operation and performance of a pressurised alkaline electrolyser
operating in the hydrogen fuelling station in Porsgrunn, Norway. Energy Convers Manag 2015;94:40–50. http://dx.doi.org/10.1016/j.
enconman.2015.01.070.
[4] Chen M, Chou SF, Blaabjerg F, Davari P. Overview of power electronic converter topologies enabling large-scale hydrogen production
via water electrolysis. Appl Sci (Switzerland) 2022;12(4):1906. http://dx.doi.org/10.3390/app12041906.
[5] Øystein U. Modeling of advanced alkaline electrolyzers: A system simulation approach. Int. J. Hydrogen Energy 2003;28(1):21–33.
http://dx.doi.org/10.1016/S0360-3199(02)00033-2.
[6] Alfredo U, Sanchis P. Static-dynamic modelling of the electrical behaviour of a commercial advanced alkaline water electrolyser. Int.
J. Hydrogen Energy 2012;47(64):18598–614. http://dx.doi.org/10.1016/j.ijhydene.2012.09.125.
[7] Energinet. Regulations for grid connection. Retrieved from https://en.energinet.dk/Electricity/Rules-and-Regulations/Regulations-for-grid-
connection.

483