Embodiments of the present invention relate to a submodule for a power converter that has a short-circuit device, and a power converter with a number of such series-connected submodules.
Power converters for conversion of direct current into alternating current and vice versa and for many other purposes are frequently built of submodules that have a bridge circuit with controllable power semiconductor switches and an internal capacitor for temporary storage of electrical energy that forms a direct voltage intermediate circuit. The bridge circuit is connected in parallel with the intermediate circuit and can be formed, e.g., by a half bridge with two controllable power semiconductor switches that are connected in series. The bridge circuit can also be a so-called H-bridge or full bridge that has two parallel power semiconductor branches connected to the intermediate circuit, each of which has two power semiconductor switches arranged in series in it. Each controllable switch has an anti-parallel freewheeling diode assigned to it. The power semiconductor switches are controlled in a suitable way to produce an alternating voltage at the connection points of the switches, for example. Conversely, alternating voltage at the connection points of the switches can be converted into direct voltage to feed the intermediate circuit.
For high-power power converters, multiple such submodules are usually connected in series with one another to allow high rated system power in the megawatt range and high-voltage applications such as high-voltage direct current (HVDC) transmission systems, and to produce largely sinusoidal voltages with fine gradation. Redundancy requirements, according to which the function of a device must be ensured even when one or more submodules fail, can also require the use of multiple series-connected power converter submodules.
It is important that a failed submodule does not, to the extent possible, affect the functionality of the entire system. A fault can be caused by a damaged or destroyed power semiconductor switch or a damaged or destroyed freewheeling diode of the bridge circuit or also by a failure in the driver of the power semiconductor switch. Various semiconductor switches are available for use. E.g., the submodules can be built using insulated-gate bipolar transistors (IGBTs) in so-called flat-pack or modular design that have multiple IGBT and anti-parallel diode chips on a substrate. Each chip is electrically connected through bond wires with the module-internal busbar. When a fault occurs, a short circuit current can cause an IGBT chip to break down and form a short circuit between its collector and its emitter connection. The defective chip then usually conducts the entire fault current, whose intensity and the overheating associated with it can lead to the melting or tearing off of the bond wires within microseconds. This can result in arcing faults that can lead to an explosion of the IGBT module in question and other IGBT modules, and finally open the electric circuit of the entire submodule. This would interrupt the current in the series circuit of the submodules, which would have the consequence of shutting down the entire power converter. This should be prevented. When a fault occurs, it may be beneficial to set up a long-term low-impedance current path between the AC voltage connections of the faulty submodule, to allow redundancy between the submodules and ensure the further functionality of a power converter or the entire system.
In one process used to short-circuit a faulty submodule of a power converter, the submodule is connected with a full bridge circuit and with at least one internal intermediate circuit capacitor as energy storage in a series circuit of submodules, all power semiconductor switches being controlled in such a way when a fault occurs that they permanently break down to create a durable short-circuit of the direct voltage side of the submodule. When a fault occurs, the power semiconductor switches are destroyed, which can be costly. Implementing the process to achieve, on the one hand, a quick breakdown of the power semiconductor switches and simultaneously avoid the formation of arcing faults and explosion of the submodule components, which are manufactured in a modular design, can be very complicated and in many cases also scarcely achievable.
In another aspect, a short circuit for a faulty power converter submodule has an internal intermediate circuit capacitor and a full bridge circuit wherein each intermediate circuit capacitor has an electronic semiconductor device connected in parallel with it that, when a submodule fault occurs, either accepts a short circuit current of the intermediate circuit capacitor or, depending on such a short circuit current, is controlled and then permanently breaks down, or breaks down as a consequence of an excessive capacitor voltage. The semiconductor device connected in parallel with the intermediate circuit capacitor can be a diode, a short-circuit thyristor, or a power semiconductor switch, in particular an IGBT. When a fault occurs, e.g., the short-circuit thyristor is fired, and the power semiconductor switches of the bridge circuit are controlled in such a way that they desaturate and the short circuit current quickly commutates to the fired short-circuit thyristor, which then breaks down and forms a durable short-circuited bypass path. Such a bypass branch with the associated semiconductor device requires additional components and increases the complexity of the circuit.
It is also usual in practice today to provide so-called AC short-circuiters that are formed, for example, by fast-switching mechanical switches that are arranged in a bypass branch between the AC voltage connections of the submodule and are closed when a fault occurs to create a short circuit bypass path between the AC voltage connections. Here again, additional components are required for the bypass path.