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Description

The MMC cell block implements a unipolar bridge with a capacitor. Series RC snubber circuits are connected in shunt with each switch device. Press Help for suggested snubber values when the model is discretized. The gates are controlled by Double signals. The following figure presents the equivalent electrical circuit of the MMC cell block implementing a unipolar bridge.

When the upper switch or upper anti-parallel diode conducts, the voltage between the Center and the Common equals Vc (minus internal voltage drops). When the lower switch or diode of the leg conducts, this voltage is equal to 0 (plus internal voltage drops).

The RC snubber in shunt with the switch are required to solve numerical oscillation. Using the time step and the equivalent inductance of the circuit the value of the Rsnubber and Csnubber are given by the following equation.

$$\mathrm{R}_{\mathrm{Snubber}}=\frac{\pi}{\mathrm{Ts} \cdot 5} \cdot \mathrm{L}_{\mathrm{eq}}$$ $$\mathrm{C}_{\mathrm{Snubber}}=\frac{1}{\left(\mathrm{L}_{\mathrm{eq}} \cdot \frac{2 \cdot \pi}{\mathrm{T} \mathrm{s} \cdot 15}\right)^{2}}$$

Where Leq is the equivalent inductance and Ts the simulation time step.

The resistance in shunt with the capacitor will discharge it. If no discharge resistance is required, it should be set to inf.

If the upper and lower switches are turned ON at the same time, the error output will output 1 when the error occurs and 0 otherwise. Upon error the capacitor voltage will become 0 instantaneously and return to normal once the fault is cleared. If the “keep cell short-circuit upon error” is checked the capacitor voltage will always remain 0. The error output will indicate the total number of short-circuit cells.

Snubber resistance Snubber resistance value, only used in high impedance mode. Snubber capacitor value, only used in high impedance mode. Value of the cell's capacitor. Value of the discharge resistance for the cell's capacitor. The initial voltage value of the cell capacitor. It can be a scalar if all the capacitors start at the same value. If a vector the same length then the total number of cell is used, the voltage for each cell’s capacitor can be set. The internal resistance of the selected device, in ohms. This determines how many cells are connected in series. A maximum of 50 cells can be connected in series. If more then 50 cells are required, a second MMC_1P block needs to be connected in series. Time at which the capacitor voltage will be computed. Check this box if the cell capacitor should remain to 0 when an error occurs.

Inputs and Outputs

Inputs

Upper gate (double) double signals that controlled the upper switch gates. This signal has to be a vector of the same length as the number of cells. A signal value of 1 indicates the switch is conducting, while a value of zero indicates the switch is OFF. double signals that controlled the lower switch gates. This signal has to be a vector of the same length as the number of cells. A signal value of 1 indicates the switch is conducting, while a value of zero indicates the switch is OFF. Middle point of the cell. Common point of the cell.

Outputs

Vc (double) The voltage at the cell's capacitor, vector of same length then the number of cells. Ouput 0 in normal operation and output 1 if a short-circuit occurs. When the “keep cell short-circuit upon error” is checked it will output the total number of short-circuited cells.

Characteristics

Direct Feedthrough No Parameter Yes Yes
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