Modular voltage source converter

1. A modular voltage source converter comprising one or more phases, each of said phases comprising one or more converter cell modules connected in series to each other, each of said converter cell modules comprising a first and a second phase leg with self-commutated valves, an output voltage of said voltage source converter is controlled by control signals applied to said converter cell modules, each converter cell module haven an output voltage AC terminal(U AC ).

wherein in case of a failure in the phase leg of the converter cell module, said failed converter cell module is controlled only by the control signals controlling the valves of the second, non-faulty phase leg of said failed converter cell module, such that a zero output voltage is provided at the output voltage AC terminal (U AC ).

2. The voltage source converter of claim 1 , wherein an additional number of converter cell modules are provided in each phase to ensure trip free operation during service interval.

3. The voltage source converter of claim 1 , wherein each of said converter cell modules comprises four valves arranged in a full-bridge connection, and each of said valves is of a press pack design wherein the valve, upon a failure, enters a short circuit mode.

4. The voltage source converter of claim 1 , wherein each of said converter cell modules comprises four valves arranged in two half-bridge connections, and wherein said half bridge connection comprises two of said valves.

5. The voltage source converter of claim 3 , wherein each of said converter cell module is assigned a gate unit being physically connected to the valve, said control signals are valve turn on and turn off signals applied via said gate unit, and said gate unit is adapted to receive detection signals from the cell module.

6. The voltage source converter of claim 1 , wherein each valve comprises an insulated gate bipolar transistor (IGBT) with an anti-parallel diode.

7. The voltage source converter of claim 1 , further comprising a cell control and protection unit arranged to generate a fault signal if any fault is detected in any of said converter cell modules, said fault signal is applied to a control and protection device responsible for the overall control of the voltage source converter.

8. The voltage source converter of claim 7 , wherein the total output voltage from the converter is maintained by controlling, by said control signals, non-faulty cell modules to increase their output voltages to compensate for zero output voltage from one or more faulty cell modules in accordance with a fault handling control algorithm.

9. The voltage source converter of claim 8 , wherein said fault handling control algorithm is applied by said control and protection device when a fault signal is detected.

10. The voltage source converter of claim 6 , wherein if the diodes in a faulty phase leg are healthy and the IGBTs are not short circuited, the other phase leg of a faulty cell will then be operated at fundamental switching frequency.

11. The voltage source converter of claim 6 , wherein if one IGBT is short circuited, then the zero output voltage will be achieved by only switching the IGBT in the other phase leg, opposite to the short circuited IGBT, while keeping the other IGBT in that healthy phase leg turned off resulting in that current will only flow in the IGBT in the other phase leg, opposite to the short circuited IGBT, and the short circuited IGBT.

12. The voltage source converter of claim 1 , wherein said three phases are connected in a delta configuration.

13. The voltage source converter of claim 1 , wherein said three phases are connected in a Y configuration.

14. The voltage source converter of claim 1 , wherein protective covers are arranged such that one protective cover is provided to enclose two valves of a cell module in order to protect the environment in case of fire or explosion, the protective cover is essentially a box having rectangular surfaces having an open bottom surface and encloses completely the two valves.

15. A method in a voltage source converter based on a converter cell topology, said converter comprising one or more phases, each of said phases comprising one or more series-connected converter cell modules connected to each other, each of said converter cell modules comprising a first and second phase leg with valves, an output voltage of said voltage source converter is controlled by control signals, generated by a control and protection device, applied to said converter cell modules, each converter cell module having an output voltage AC terminal (U AC ),wherein the method comprises the following steps:

A) detecting a failure in the first phase leg of a converter cell module,

B) analysing the failure,

C) determining a failure type, and

D) controlling, in dependence of the failure type, the valves of the second non-faulty phase leg of the converter cell module such that a zero output voltage is provided at its output voltage AC terminal (U AC ).

16. A modular voltage source converter comprising one or more phases, each of said phases comprising one or more converter cell modules connected in series to each other, each of said converter cell modules comprising a first and a second phase leg with self- commutated valves, an output voltage of said voltage source converter is controlled by control signals applied to said converter cell modules, each converter cell module having an output voltage AC terminal (U AC ),

wherein in case of a failure in the phase leg of a converter cell module, said failed converter cell module is controlled only by the control signals controlling the valves of the second, non-faulty phase leg of said failed converter cell module, such that a zero output voltage is provided at the output voltage AC terminal (U AC ), and

wherein for each converter cell module four external diodes are arranged in parallel to each of the valves of the module, said external diodes are arranged separately from, and are protected from the cell module, and said external diodes are used upon failure of said cell module(s).