JP7347161B2 - power converter - Google Patents

Description

The present invention relates to a power conversion device in which cell converters are multiple-connected in series, and relates to a technique for bypassing the cell converters.

A modular multilevel cascade converter (MMC) is a power converter suitable for large-capacity, high-voltage applications. MMC can be applied to, for example, a reactive power compensator (STATCOM) or a direct current power transmission system (HVDC).

The MMC circuit system is presented in Patent Document 1, for example. Figure 8 shows the MMC main circuit configuration. In FIG. 8, con. 1, con. 2, con. 3 are cell converters. These cell converters are connected in series in multiple stages to form a cluster. In the example of FIG. 8, the clusters of each phase (Theu-Phase Cluster, The v-Phase Cluster, The w-Phase Cluster) are three-phase Y-connected. One cell converter includes a bridge circuit in which a plurality of switching elements are bridge-connected, and a capacitor connected to a DC bus of the bridge circuit. Capacitors Cu1, Cu2, and Cu3 in each cell converter are controlled so that their respective voltages vCu1, vCu2, and vCu3 are equal during normal operation. As a result, the high voltage equivalent to the phase voltage of the three-phase power system applied to each phase cluster is divided to each cell converter in the cluster, so that the cell converter is applied to the voltage of the three-phase power system. On the other hand, it can be constructed from components with relatively low withstand voltage, and its multistage connection allows it to be applied to high-voltage applications. In FIG. 8, switches MC1, MC2 and resistor R constitute a device for preventing inrush current at the time of starting the MMC, Lac represents a current smoothing reactor, and Ls represents a reactance component of the system.

Although FIG. 8 shows a Y-connection MMC, a delta-connection is also possible.

A method is known in which when a cell converter fails, the output of the cell converter is short-circuited and bypassed to continue operating the power converter. For example, if a switch with two thyristors connected in anti-parallel is used as an output short-circuit switch (hereinafter referred to as a bypass switch), and when a cell converter fails, an on signal is input to the two semiconductor switches and the cell converter is switched on. Short the output. By short-circuiting the output of the failed cell converter, it is possible to prevent the operation of the cell converter from affecting the operation of the MMC. On the other hand, a cluster including short-circuited cell converters divides the phase voltage using cell converters that are not short-circuited, and functions as a cluster in which the number of series-connected cell converters is reduced by one.

Patent Document 2 proposes a method using a pressure contact type semiconductor element as a bypass switch. FIG. 9 shows the cell converter configuration of Patent Document 2. The output terminal of the cell converter is equipped with a press-contact type thyristor (999) as a bypass switch. By applying a voltage higher than the thyristor's withstand voltage to this press-contact type thyristor, it will break down due to a short circuit, and the output of the cell converter will be short-circuited. The voltage applied at this time is the DC capacitor voltage of the cell converter.

JP2017-163765 JP2013-027260

In the method described in Patent Document 2, a bypass function is achieved by applying a DC capacitor voltage of a cell converter to a voltage higher than the withstand voltage of the press-contact type semiconductor element to destroy it. However, a large current can flow at the moment a pressure contact type semiconductor element is short-circuited due to overvoltage, which may cause the device to stop due to activation of the MMC device's protection function, or in the worst case, cause other cells through which the large current flows to There was also the problem that the converter could malfunction.

Therefore, the problem to be solved by the present invention is to perform short-circuit destruction of a pressure-contact type semiconductor switch by passing current, thereby reliably short-circuiting the output of a cell converter when necessary, without causing the above-mentioned problems. It is to be.

In order to achieve the above object, a power converter according to the present invention is a power converter in which three clusters each having the same number of cell converters connected in series constitute a delta connection, and each of the cells The converter includes a main circuit that is composed of a switching element and a capacitor and generates a rectangular wave voltage, a pair of external output terminals that output the voltage, and a bypass switch connected between the pair of external output terminals. and the plurality of cell converters are configured such that the external output terminals of each cell converter are connected in series, and the bypass switch is connected to the cell converter other than the cell converter equipped with the bypass switch. It is characterized in that it is always in a short-circuit state by flowing a predetermined current .

According to the present invention, by passing a predetermined current through the bypass switch and always keeping the bypass switch in a short-circuited state, it is possible to stably and reliably perform a bypass operation when a cell converter fails.

MMC configuration diagram Block diagram of a cell converter with a bypass switch in MMC Bypass switch configuration example Status of each switch when the bypass switch in the cell converter is always short-circuited Schematic diagram showing voltage sharing when one of the cell converters between the UV lines is short-circuited in a delta-connected MMC Status of each switch when the bypass switch in a cell converter is constantly shorted by another cell converter Configuration diagram with a power receiving switch between the power system and MMC Explanatory diagram of MMC configuration Block diagram of a cell converter with a bypass switch in MMC

Embodiments of the present invention will be described below.

First, a delta connection type MMC will be explained as an example. Figure 1 shows the circuit configuration of a delta connection type MMC.

The delta connection type MMC consists of a cluster (50UV, 50VW, 50WU), which is formed by connecting multiple unit converters called cell converters (500UV1 to 500UVx, 500VW1 to 500VWx, 500WU1 to 500WUx) in series, and a reactor (51UV, 51VW, 51WU). ) are connected in delta connection. The voltage across the cluster is the sum of the rectangular wave voltages generated at the output terminals of the cell converters included in the cluster, so each cell converter outputs a voltage waveform with a different phase, reducing harmonics. Multi-level waveforms can be synthesized.

Figure 2 shows the configuration of the cell converter. The cell converter (500) has a so-called full-bridge circuit in which a plurality of switching elements (501 to 504) are bridge-connected, and is a full-bridge inverter in which a capacitor (505) is connected to the DC bus of the full-bridge circuit. configured. A pressure contact type semiconductor switch is connected between the output terminals of the cell converter as a bypass switch (511) for short-circuiting and bypassing the cell converter. Generally, the failure mode of press-contact type semiconductors is short-circuit failure, so they are suitable for the bypass application according to the present invention. Power is supplied to the GDU (506-508), which is a gate drive circuit that drives each switching element (501-504), through a self-power supply circuit (510) connected in parallel with a capacitor (505). The DC voltage held by the capacitor supplies power to the self-power supply circuit (which has low power consumption because it is an electronic circuit), and the necessary power is shared by each GDU from the self-power supply circuit. Each GDU provides an on/off signal to a corresponding switching element in accordance with a switching command value given from a control unit (not shown). This operation generates a rectangular wave voltage at the output terminal of the cell converter. Note that in FIG. 2, only the connection of the bypass switch to the GDU is schematically shown.

FIG. 3 shows a configuration example of a bypass switch (511) using a pressure contact type semiconductor switch. There is a configuration in which two thyristors (511a, 511b) are connected in anti-parallel as shown in Figure 3(a), and a configuration in which two IGBTs (511c, 511d) are connected in anti-series as in Figure 3(b). . When not bypassing, each element is turned off; when bypassing, each element is turned on, and a predetermined current is applied to destroy the element, thereby creating a constant short-circuit state. Device destruction due to current flow is generally caused by a rise in the temperature of the semiconductor, so the temperature of the semiconductor that leads to destruction, as well as the current value and conduction time to reach that temperature, should be understood at the time of design, and the current flow should be adjusted accordingly. All you have to do is decide on the current to flow and the duration of the flow.

An example of a method for passing a predetermined current through a bypass switch will be described with reference to FIGS. 4(a) and 4(b). Turn on the switches (501, 504) and the bypass switch (511) and turn off the switches (502, 503) as shown in FIG. 4(a), or turn off the switches (501, 504) as shown in FIG. 4(b). By turning on the switches (502, 503) and the bypass switch (511), the accumulated charge in the capacitor (505), which holds a predetermined voltage during normal operation, flows through the bypass switch (511). The procedure is to first turn on the bypass switch, and then turn on the predetermined switches forming the bridge so that a predetermined current flows through the bypass switch. In this case, by making the switches (501 to 504) forming the bridge stronger than the bypass switch (501), the bypass switch always becomes short-circuited first. After a predetermined current is caused to flow through the bypass switch so that the bypass switch is always in a short-circuited state, the elements constituting the bridge may be turned off. Alternatively, after the bypass switch is always short-circuited, the bypass switch secures a path for the current to flow through the cluster, so it is possible to discharge the charge in the capacitor without turning off the switches that make up the bridge. be. Note that when passing a predetermined current through the bypass switch in this manner, it is also possible to control the average current by rapidly turning on and off the switches forming the bridge.

The above-described method of always shorting the bypass switch can be applied to either delta connection or Y connection MMC because its operation is completed with one cell converter.

Next, a cell converter bypass means specialized for delta connection type MMC will be explained. FIG. 5 shows the overall configuration of the delta-connected MMC, and FIG. 6 shows the state of the cell converter 500UV1 shown in FIG. 5. In FIG. 5, in order to simplify the diagram, the number of cell converters connected in series in one cluster is three, and the notation of a reactor and an MMC power system is omitted. That is, in the cell 500UV1 among the three cell converters between the UV lines, the bypass switch is in the on state, and the switch forming the bridge is in the off state. At this time, the effective value of the fundamental wave of the line voltage output by each cluster is made to be equal. In other words, as shown in the figure, the number of operable cells between the UV lines is 2, and the effective voltage values between the VW lines and the WU lines are added to the effective voltage value V1 × 2 generated by these cells. In order to match, the effective voltage value of each cell converter connected between these lines may be set to V1×2÷3. At this time, it is possible to control the circulating current circulating through the delta connection, and thereby destroy it by setting the current flowing through the bypass switch (511) of the cell converter 500UV1 to a predetermined value. A method of controlling circulating current in a delta connection type MMC is a known technique, and is described, for example, in the patent publication No. 5800154. Although the case where the number of cells in each phase is three has been described here, the present invention is not limited to this, and can be applied to any number of cells. With the technology described here, even if the capacitor cannot be short-circuited as described above due to an open failure of the switch in the cell converter, the specified current can be passed through the bypass switch of the cell converter. It is possible to keep the current short-circuited at all times.

Next, another embodiment of the present invention will be described. The MMC further includes a control unit (not shown), which stores the operating state of the MMC, such as the voltage and current flowing through each switching element, the voltage of each capacitor, and the usage time of each part, and detects a failure of the converter based on the operating state. The control unit includes a failure prediction unit that predicts failures. In order to reduce the influence on other cell converters, the failure prediction unit uses the method described above when predicting a failure in a cell converter (when the cell converter is healthy and the DC capacitor maintains voltage). You can short-circuit and destroy the bypass switch using For example, when an aluminum electrolytic capacitor is used as a cell converter capacitor, the life time of the aluminum electrolytic capacitor can be calculated in advance. For example, the operating history of the MMC is recorded in the failure prediction unit, and when the usage time approaches the aluminum electrolytic capacitor life time stored in advance in the failure prediction unit, the control unit can be configured to bypass the cell converter in advance. By configuring the system so that commands are output from the GDU to the GDU, secondary disasters caused by sudden damage to aluminum electrolytic capacitors can be prevented.

Next, another embodiment of the present invention will be described. If a power conversion device is operated to cause damage by short-circuiting the bypass switch, the effect will extend to the power system, causing system disturbances and potentially damaging other equipment. Therefore, as shown in Fig. 7, a power receiving switch (6) is provided between the power system (1) and the power converter (2), and the power receiving switch (6) is turned off to transfer the power from the power system (1) to the power converter ( By performing a destructive operation on the bypass switch after disconnecting 2), it is possible to prevent the impact on the power system. Even if the power converter is disconnected from the power system, if the capacitor of each cell converter maintains the specified DC voltage, it is possible to keep the bypass switch in the short-circuit state using the cell converter alone as described above. Similarly, as described above, if the power converter is a delta connection type MMC, a circulating current path is secured by the delta connection, so a method using circulating current can also be adopted.

1...Power supply system 5...MMC
50UV, 50VW, 50WU...Cluster 500UV1 to 500UV3,500UVx, 500VW1 to 500VW3,500VWx, 500WU1 to 500WU3,500WUx...Cell converter 51UV, 51VW, 51WU...Reactor 501 to 504...Switching element 505 forming the cell converter …capacitor 506-509...GDU (gate drive circuit)
510... Self-power supply circuit 511... Bypass switch 512... GDU for bypass switch
511a, 511b...Thyristor 511c, 511d...IGBT
6...Power receiving switch

Claims (3)

A power conversion device in which three clusters each having the same number of cell converters connected in series constitute a delta connection,
Each of the cell converters is connected between a main circuit that is composed of a switching element and a capacitor and generates a rectangular wave voltage, a pair of external output terminals that output the voltage, and the pair of external output terminals. Equipped with a bypass switch,
The plurality of cell converters have the external output terminals of each cell converter connected in series,
The bypass switch is a cell other than the cell converter equipped with the bypass switch.
A power converter characterized by being constantly in a short-circuit state by flowing a predetermined current from a converter . A power conversion device according to claim 1,
A power conversion device characterized in that the bypass switch has a smaller current withstand capacity than the switching element. A power conversion device according to any one of claims 1 or 2,
The power conversion device is connected to a power system via a power receiving switch,
A power conversion device characterized in that the predetermined current is caused to flow through the bypass switch while the power receiving switch is turned off.