Ground Fault Diagnosis and Protection of Modular Multilevel Con verters (DIAPROCON [233])

The Modular Multilevel converters are gaining momentum in power converter-driven power sys
tems. Their reduction in the Total Harmonic Distortion (THD) and their flexibility of control make 
them very versatile compared to conventional two-level converters. However, as it happens with 
any power converter-based system, they are very exposed to ground faults due to their acceler
ated aging process caused by the semiconductors switching. 
Ground faults (L-G) are caused due to insulation degradation. Depending on the grounding to
pology of the electric systems these faults can be more or less harmful and more or less detect
able. In case of isolated grounding configurations, a first ground fault does not affect the opera
tion of the system. However, a second fault will incur into a Line-to-Line (L-L) fault, which will 
harm the system considerably. Also, in case of a rigid grounding, the L-G faults have also high 
fault currents which will damage also the system. However, its detectability increases due to this 
fact.  
In this technical report, the experimental validation of a ground fault location method developed 
by Jose M. Guerrero et al. (2023) is described. This method uses a DC midpoint high resistance 
grounding configuration, and some voltage measurements to detect the fault, discern the phase 
in fault and locate the module in fault. In this case, the grounding configuration proposed is an 
intermediate case between isolated and rigid groundings. This fact enables the normal opera
tion condition of the system with a first fault as the fault current is limited by the grounding resis
tor. Afterwards, with the abovementioned voltage measurements, the fault is detected if the 
grounding voltage is up to a certain Root Mean Square (RMS) value. The faulty phase is dis
cerned comparing the phase angle between the AC voltages and the grounding resistor voltage. 
Finally, the location inside the arm is located considering the relative value of DC component 
respecting to the AC main harmonic in the grounding resistor voltage. The DC bus voltage 
measured will be recorded with severity estimation purposes in further works.  
The experimental validation was carried out in the SINTEF laboratories, that have been used 
under the EriGrid 2.0 project. The experimental setup used was a 60 kVA 12-levels three-phase 
Modular Multilevel Converter (MMC) with intermediate terminals that allow performing ground 
faults inside the submodules (SM). 
More than 400 tests were conducted from the June 6th to the June 13th 2024 in order to validate 
the method. The faults considered involve all the fault positions (24 submodules, DC positive 
and negative poles and AC side fault) for the three phases and for fault resistances of 0, 2.2, 
4.4, 6.6, 8.8, and 11 kΩ. The ground faults have been performed utilizing a galvanically isolated 
ideal grounding in order to not affect the rest of the laboratory during the tests. The results are 
promising. They allow to detect the submodule in fault with an accuracy of ±1 submodule in the 
intermediate zone, which can be admissible in a 24 submodules-per-phase MMC. Also, the 
experimental validations have allowed the user group to realize about the non-linearities of the 
problem, due to not all the capacitors involved in the fault circuit are equalized. Experiments 
have proven that voltage differences among capacitors can be up to 1 V, which can finally lead 
to a total of 25 V of difference between faults, i.e., an induced error of a submodule: 4.16%.  
With the experimental data collected, further works should aim to obtain a deeper theoretical 
understanding about the location method in real MMCs. Furthermore, additional methods and 
intellectual property can be generated.