Patent ID: 12261438

DETAILED DESCRIPTION OF THE INVENTION

FIG.1shows a device for controlling a power flow in an AC network2(e.g. a power supply network), wherein the AC network comprises an AC link with three phase lines2a,2band2c. The device1is a universal power flow controller (UPFC). The device1comprises a series converter3with an AC side4and a DC side5. On its AC side the series converter3has three AC terminals4a,4b,4c. The series converter3is configured to be (and under operation condition is) connected to the AC network2via a series transformer6. The DC side5of the series converter3has two DC terminals5a,5b(e.g. positive and negative terminals) configure do be connected to a DC link7. The series converter3is a modular multilevel converter (MMC). It comprises three phase modules8a-cand six converter arms (valves)9a-f. Every converter arm9a-fextends between one of the DC poles or terminals5a,band one of the AC terminals4a-c. Each converter arm9a-fcomprises an arm inductance L and a number of switching modules10connected in series. In accordance with the embodiment shown inFIG.1all switching modules are configured alike, which however is not necessary in general. The number of switching modules10in every converter arm9a-fis in general arbitrary (not restricted to three as shown in the figure) and can be adapted to the given application. According to the example ofFIG.1the switching modules10are so-called half-bridge switching modules. The switching module10comprises two terminals X1, X2to connect e.g. to further, neighboring switching modules. The switching module10further comprises two semiconductor switches11,12of the turn-off type with a freewheeling diode D in antiparallel. An energy storage element (capacitor)13is connected in parallel to the series connection of the semiconductor switches11,12. By a proper control of the switches11,12a voltage across the terminals X1, X2can be achieved which equals to the voltage of the capacitor13or a voltage (substantially) equal to zero. Instead of a half-bridge circuit, any (or even all) of the switching modules can comprise any other suitable circuit. An example is the full-bridge configuration known from the prior art.

The device1further comprises a shunt converter14which is a voltage source converter (e.g. an MMC). The shunt converter14is on its AC side16connectable (connected in normal operation) to an AC network (e.g. the AC network2) via a shunt transformer15. The shunt converter14is furthermore connected, on its DC side17, to the series converter3via the DC link7. A startup circuit18is provided between the shunt transformer15and the shunt converter14.

A central control unit CU is provided and configured to control the series converter3and the shunt converter14by means of controlling the respective semiconductor switches.

The series transformer6comprises primary windings19a-cwhich are connected in series with the respective phase lines2a-cof the AC link2. Each of the primary windings19a-ccan be bypassed by a respective bypass switch (a circuit breaker)20-22. The series transformer6further comprises three secondary windings23a-cconnected to each other in a delta connection forming three delta branches24a-c.

In addition, the device1comprises a bridging arrangement25to bridge the series converter3in case of a fault. The bridging arrangement25comprises three bridging branches26a-ceach connected in parallel to one of the delta branches24a-c. A first bridging branch26acomprises a switching unit27with two antiparallel thyristors28a, b. The first bridging branch26acomprises further a resistance29(a resistor element) and an inductance30arranged in series with the switching unit27. A MOV-arrester31is provided in parallel with the series circuit of said resistance29, inductance30and switching unit27. The second bridging branch26band the third bridging branch26care arranged in a similar manner.

InFIG.2a flow diagram shows a method of operating a device for controlling power flow in an AC network, for example the device ofFIG.1. Said method of operation comprises in particular a recovery sequence of the series converter in case of a fault. The embodiment ofFIG.2shows the method steps performed in case of an external fault. In the following all numerals referring to aspects of the device correspond to those used inFIG.1.

At a time t0 a fault occurs and is detected by a suitable detection device. The series transformer6is protected against high voltage by arresters31.

At a time t1, ca. 1-50 microseconds after t0, the converter is actively blocked, e.g. via a converter current protection.

At a time t2, ca. 1.5-2 ms after t0, the thyristors28a,bof the bridging arrangement25are actively switched on (‘fired’). Accordingly, the series converter is protected via the bridging arrangement25. The fault current is commutated from the converter to the bridging branches26a-c(in particular through the thyristors) and flow continually through said bridging branches until the fault is cleared. Transients occurring at t2 are largely suppressed due to the presence of the resistor elements29in the bridging branches.

At a time t3, ca. 20-30 ms after t0, the series converter is actively deblocked. Before deblocking of the series converter a valve current through the thyristors and a converter current through the series converter are measured and compared with a respective threshold in order to decide whether the deblocking can be initiated.

At a time t4, approx. 50-150 ms after t0, the fault is cleared via a line circuit breaker. From t4 on a normal line current will flow through the bridging branches. The balancing of the series converter3is actively started.

At a time t5 the thyristors are blocked (achieved at a current zero crossing by not actively providing a firing pulse). The line current is commutated from the bridging branches to the series converter3. Transients occurring at t5 are largely suppressed due to the presence of the resistor elements29in the bridging branches.

At a time t6, approximately 100-200 ms after t0, the series converter returns to normal operation.

InFIG.3a flow diagram shows a method of operating a device for controlling power flow in an AC network, for example the device ofFIG.1. Said method of operation comprises in particular a recovery sequence of the series converter in case of a fault. The embodiment ofFIG.3shows the method steps performed in case of an internal fault. In the following all numerals referring to aspects of the device correspond to those used inFIG.1.

At a time s0 a fault occurs and is detected by a suitable detection device. The series transformer6is protected against high voltage by arresters31.

At a time s1, ca. 1-50 microseconds after s0, the converter is actively blocked, e.g. via a converter current protection.

At a time s2, ca. 1.5-2 ms after s0, the thyristors28a,bof the bridging arrangement25are actively switched on (‘fired’). Accordingly, the series converter is protected via the bridging arrangement25. The fault current is commutated from the converter to the bridging branches26a-c(in particular through the thyristors) and flow continually through said bridging branches until the fault is cleared. Transients occurring at s2 are largely suppressed due to the presence of the resistor elements29in the bridging branches.

At a time s3, ca. 35-50 ms after s0, the bypass switches20-22is switched on, so that the series converter3and the series transformer6are both protected via the bypass switches. The fault current is commutated from the bridging branches to the bypass switches and flows continually through the bypass switches until the fault is cleared.

At a time s4 the thyristors28a,bin the bridging branches are blocked (this can be achieved at a current zero crossing by not actively providing a firing pulse). The line current is commutated from the bridging branches to the series converter3.

At a time s5, approx. 50-150 ms after s0, the fault is cleared via a line circuit breaker.

At a time s6, ca. 300-1000 ms after s0, the AC line is energized, and a nominal power is transmitted.

At a time s7, the series converter3is actively deblocked, the thyristors in the bridging branches are actively fired and opening of the bypass switches20-22is initiated (in the indicated sequence). Before deblocking of the series converter a valve current through the thyristors and a converter current through the series converter are measured and compared with a respective threshold in order to decide whether the deblocking can be initiated.

At a time s8 the line current flows through the bridging branches26a-cand no current flows through the bypass switches20-22. This is confirmed to the central control unit CU by a suitable measurement set up. The balancing of the series converter3is actively started.

At a time s9 the thyristors of the bridging arrangement are blocked. Transients occurring at s9 are largely suppressed due to the presence of the resistor elements29in the bridging branches.

At a time s10, approximately 100-200 ms after s0, the series converter returns to normal operation.