# 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.