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VSC Solid State Transformer


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

ParametersDescriptionValue
Pref_DABDAB power rating per module20 kW
VmvdcMedium voltage-side dc voltage per module4 kV
VLvdcLow voltage-side dc voltage400 V
ftranDAB transformer frequency2 kHz
Ls_DABDAB external inductance0.0420 H
Vac_MVMedium voltage-side ac voltage8.5 kV
freqMedium and low voltage-side grid frequency60 Hz
CC_mvdcMedium voltage-side capacitance per module0.125 μF
Vac_LVLow voltage-side ac voltage141.4 V
CC_lvdcLow voltage-side capacitance11.3 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 transformer583.78 s16.54 s35.29

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 transformer2907.6 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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