Location

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

Description

This model simulates a back to back voltage source converter (VSC) connecting an ac grid and an induction machine. The types of VSC converters include two level and three level converters (neutral-point clamped (NPC) converter and T-type converter).

There are also two types of topologies for the same converter depanding on whether the midpoint of dc link is connected to ground. All the possible topologies are listed as following

(a) two level converter without GND (b) two level converter with GND

(c) NPC converter without GND (d) NPC converter with GND

(e) T-type converter without GND (f) T-type converter with GND

Fig. 1 Topologies of VSC converters

The VSC is modelled by equivalent circuit method presented in [1] and defined as OPAL model in this demo. A model with detail switches is built in SPS and used as the reference. The subsystem "SM_Conv" contains all the OPAL and SPS models as listed in Fig. 1, which includes 12 converters totally. The specific converter can be selected for simulation by using Variant Subsystem block in Matlab. The steps for converter selection are listed as

Step 1: select SPS or OPAL subsystem

In the initial file, define the value of V_MODEslv. OPAL model is selected if V_MODEslv=0 and SPS model is selected if V_MODEslv=1.

In the RTLAB model, the selected model will be indicated inside the subsystem "SM_Conv"

Step 2: select converter types

In the initial file, define the value of V_model. Two level converer is selected if V_model=0, NPC converter is selected if V_model=1, and T-type converter is selected if V_model=2.

In the RTLAB model, the selected conveter will be indicated inside the subsystem "OPAL" or "SPS" depanding on which one is selected in Step 1.

Step 3: select topologies with or without GND

In the initial file, define the value of V_GND1. The topology without GND is selected if V_GND1=0, The topology with GND is selected if V_GND1=1.

In the RTLAB model, the selected conveter will be indicated inside the subsystem of the converter selected in Step 2.

Step 4: select submodule type

In the subsystem "SC_Console" of the RTLAB model, select the submodule types in the conv_para block (VSC Parameter Block). For a three-phase back to back system, the nubmer of SM should be set as 6.

Select "Half Bridge SM" for two level converter

Select "NPC (neutral point clamped)" for NPC converter

Select "T-type SM" for T-type converter

Simulation and Results

To illustrate the accuracy of the modeling method, the OPAL model (SWF) with 50 µs is compared with the SPS model with 1 µs. The comparison results are presented. The results validate that the OPAL model with 50 µs has matched performance as the SPS detail model with 1 µs. The System parameters are listed as

Parameters	Description	Value
Vdc	DC voltage	3 kV
Vprim	Grid single phase rms voltage	1.5 kV
fs	Grid frequency	60 Hz
fIM	Nominal IM frequency	100 Hz
PIM	Nominal IM Power	300 kW
VIM	Nominal IM rms LL voltage	1.5 kV
p	Poles number of IM	4
Ts	Simulation Time step	50 µs (OPAL) and 1 µs (SPS)
fcar	Carrier frequency	4878 Hz

Two level converter simulation results

NPC converter simulation results

T-type converter simulation results

Simulation performace

The actual time used for offline simulation for 10 s.

Test system	Execution time for SPS model (1us time step)	Execution time for OPAL model (50 us time step)	Acceleration factor
Half-bridge converter	810.3 s	22.6 s	35.9
T-type converter	1004.2 s	22.7 s	44.2
NPC converter	1263.5 s	22.9 s	55.2

The real-time simulation performance

Test system	Minimum time step for SPS model	Minimum time step for OPAL model
Half-bridge 2-level converter	6 us	6 us
T-type converter	12 us	6 us
NPCconverter	34 us	6 us

Control Method

The DC link voltage is controlled by the rectifier and variable voltage and variable frequency (VVVF) control is adopted for the motor drive as presented in Control System.

References

[1] W. Li and J. Bélanger, "An Equivalent Circuit Method for Modelling and Simulation of Modular Multilevel Converters in Real-Time HIL Test Bench," in IEEE Transactions on Power Delivery, vol. 31, no. 5, pp. 2401-2409, Oct. 2016.

[2] Bose, B. K., Power Electronics and AC Drives, Prentice-Hall, Englewood Cliffs, N.J., 1986.

See Also

VSC Parameter Block