Location

This example model can be found in the RTLAB under the category MMC example model with the file name "CPU_SST_acac.slx".

Description

This model simulates a three-phase solid state transformer. Each phase has 3 submodules connected with input series output parallel (ISOP) connection, shown as

To configure the model using VSC blocks, there are three steps, as follows

the first step is to set the VSC Parameter Block. The model is composed by 27 full bridge submodules (FBSMs) and 3 half bridge submodules (HBSMs).

The second step is to set the VSC Configuration Block, the total number of submodules (FBSMs and HBSMs) is 30. The number of internal capacitors is 10.
SM 1-18 are connected to the first 9 capacitors; SM 19-30 are connected to the 10th capacitor.

The total DAB number is 10, where SMs 1-9 and SMs 28-30 are not connected to the transformer, so the dab index is 0. SMs 10-27 are used in DAB converters, set DAB index 1 or -1 depends on the DAB current direction (positive for SMs on the side with current flowing into the transformer; negative for SMs on the side with current flowing out of transformer.)

The third step is to set the physical connection to the rest circuit. The first 9 SMs are in node 1-3 (SM 1-3 in node 1, SM 4-6 in node 2, SM 7-9 in node 3), and the last 3 SMs are in node 4-6.

Simulation and Results

To illustrate the accuracy of the modeling method, the OPAL model (SWF) with 20 µs is compared with the SPS model with 1 µs. The comparison results are presented. The results validate that the OPAL model with 20 µs has matched performance as the SPS detail model with 1 µs. The System parameters are listed as

Parameters	Description	Value
Pref_DAB	DAB power rating per module	20 kW
Vmvdc	Medium voltage-side dc voltage per module	4 kV
VLvdc	Low voltage-side dc voltage	400 V
ftran	DAB transformer frequency	2 kHz
Ls_DAB	DAB external inductance	0.0420 H
Vac_MV	Medium voltage-side ac voltage	8.5 kV
freq	Medium and low voltage-side grid frequency	60 Hz
CC_mvdc	Medium voltage-side capacitance per module	0.125 μF
Vac_LV	Low voltage-side ac voltage	141.4 V
CC_lvdc	Low voltage-side capacitance	11.3 mF

Solid state transformer simulation results

Simulation performance

The elapse time for 1 s offline simulation.

Test system	Execution time for SPS model (1us time step)	Execution time for OPAL model (20 us time step)	Acceleration factor
Solid state transformer	583.78 s	16.54 s	35.29

The real-time simulation performance (the minimum time step). The minimum time step is the minimum time the real time simulator needs to solve the model. This value being larger than the model discretizing time step, usually 20 ~50 us, means that the model cannot run in real time.

Test system	Minimum time step for SPS model	Minimum time step for OPAL model
Solid state transformer	2907.6 us (> model discretizing time step)	13.84 us (< model discretizing time step)

References

[1] Zhang, F., & Li, W. (2019). An Equivalent Circuit Method for Modeling and Simulation of Dual Active Bridge Converter Based Marine Distribution System. 2019 IEEE Electric Ship Technologies Symposium, ESTS 2019, 382–387. https://doi.org/10.1109/ESTS.2019.8847728

See Also

VSC Switching Function Model with Variant Subsystems, MMC STATCOM on CPU

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