Field of Invention

The invention relates to a modular converter that comprises the necessary modules (sometimes called cells or submodules) to extinguish the current produced by a DC fault and to disconnect the converter from the defective DC line. The present invention also relates to a method that performs the steps for mitigating the effects of said current with the proposed modular converter.

State of the Art

The modular multilevel converter (MMC) was disclosed by A. Lesnicar and R. Marquardt in articles such as "An Innovative Modular Multilevel Converter Topology Suitable for a Wide Power Range" and "A new modular voltage source inverter topology" in 2003. This topology usually comprises a 3-phase AC connection and a 2- node DC connection with a branch between each combination of an AC phase and a DC node. The original topology disclosed in said articles used a half-bridge circuit for each module in the branches, all of them orientated in the same direction.

However, as R. Marquardt has later stated in "Modular Multilevel Converter: A universal concept for HVDC-Networks and extended DC-Bus-applications" and in "Modular Multilevel Topologies with DC-Short Circuit Current Limitation" (dating from 2010 and 201 1 respectively) these types of modules are unable of extinguishing branch overcurrent induced by a short circuit in the DC line. As an alternative two other known topologies are presented for the modules: the full bridge and the clamp-double- submodule. Both of them allow the converter to have branch voltages which oppose the current in both directions. The clamp-double-submodule is usually better suited since the full-bridge cell requires more controllable semiconductors while not adding any special advantage (this may depend on the application).

The patent application WO2012103936 combines full-bridge modules with half-bridge modules to solve the same problem. This alternative uses less diodes than the clamp- double-submodule, although it requires the full-bridge modules capacitors to store more energy in case of a fault since only these capacitors will absorb energy when extinguishing the current.

In spite of reducing the current, both ways (full-bridge and clamp-double-submodule) require to store a certain amount of energy in the modules capacitors. This is necessary due to the branches inductances, which store energy in form of current. Upon a DC short circuit, the branch currents will increase fast. By the time the overcurrent is detected and the semiconductors are commanded to open, the energy stored by the inductances can be considerably high. This energy must be allocated among the modules capacitors so that it transforms from current energy (stored by the inductances) to voltage energy (stored by the capacitors). This means the capacitors, which play an important role during the converter normal behaviour, must be designed for this failure and may not be optimized for the normal operation. Moreover, once the converter has been disconnected from the DC line, if it is intended to continue working with the AC line (for example as STATCOM) its capacitors will be overcharged for a time.

Brief description of the invention

In view of the exposed problems, a new converter is proposed including a new type of adapting module.

The adapting module comprises a controllable semiconductor (such as an IGBT, a MOSFET or a GTO) with the corresponding antiparallel diode and a passive circuit which further comprises at least one resistor and at least one capacitor. Depending on the controllable semiconductor state, the passive circuit can be connected in series with the module terminals or bypassed. The passive circuit topology is such that when the passive circuit is bypassed, the capacitor is discharged and its energy is ultimately consumed by the resistor.

The converter comprises at least one connection to a DC line with branches formed between nodes of the DC connection and other nodes of the converter. Each of these branches comprises an inductance, a series circuit of regular modules, a regular module is generally a half-bridge module or a full-bridge module. The branches also comprise at least one adapting module. If the regular modules have a fixed polarity, then the new adapting modules are connected so that the polarity of their output voltage is opposite to that of the regular modules.

During normal operation, the semiconductors of the adapting modules are commanded to bypass the passive circuits. This way, these adapting modules do not need to be considered during the control of the converter, since they are bypassed. Any energy in the adapting modules capacitors is consumed by the resistor without interfering with the branch currents.

According to a proposed method, the currents of the branches must be periodically measured. Upon the detection of an excessive current due to a DC line fault, the semiconductors of the adapting modules are commanded to connect the passive circuits to the branches. Meanwhile, the regular modules are configured to provide an output voltage which will not increase the branch current and, when possible, will oppose said current. When this occurs, the branch current will flow through the resistors and capacitors producing a voltage which opposes the current and, consequently, lowers its value. Part of the energy the inductances store in form of current, will get stored into the proposed adapting modules capacitors, while the rest is dissipated through the resistors.

Once the branch current is sufficiently low, the converter can be disconnected from the defective DC line through a mechanical interrupter or a similar device. Afterwards, the adapting modules are commanded to bypass the passive circuits, allowing their capacitors to be discharged through their corresponding resistors and the remaining regular modules of the branch to regain the branch current control just like during normal operation.

Advantageously, only the defective DC line needs to be disconnected from the converter, so part of the service can be maintained. For example, the converter can act a STATCOM for an AC grid if it is connected to it. If the converter comprises connections to more than one DC lines, it is still possible to convert power to or from any of the nondefective DC lines after disconnecting from the defective one.

Additionally, it is proposed to measure the voltage of the defective DC line so that, once the fault has been cleared, the converter can be connected to it again.

It should be mentioned that there are two possible directions for the excessive current. The voltage of the adapting modules opposes the current that flows in the direction that corresponds to a DC line fault. A high current in the opposite direction can still be extinguished by regular modules as their capacitor voltage opposes to it. An excessive current in this direction does not correspond to a DC line fault, so it is not expected to be difficult to extinguish.

When designed, the adapting module can be optimized for the process of current extinction since it does not play any part during normal operation. However since the necessary circuit elements for the adapting module are similar to the ones of the regular module used for normal operation, it may use the same type of semiconductors or share the same heat sink as one of them. One or more modules of this type may be added to each branch depending on the design. The remaining modules can be optimized for normal operation since they are not required to be used during the current extinction. This provides more flexibility during the design of the converter. A skilled person should understand that the addition of other elements in parallel with the modules passive circuit such as a varistor or a bypass mechanical contactor, which are commonly added as a protection devices, do not change the operating principle of the adapting module. However, these elements should only bypass the module when it malfunctions or to protect it from overvoltage.

According to an advantageous further development, each branch may comprise more than one of the proposed adapting modules. The number of adapting modules can be chosen depending on factors such as the resistors resistance values, the maximum current a DC line fault can produce, the lines peak voltages and the semiconductors voltage limit.

When several adapting modules are added to the branch, it is possible (due to the component tolerances) that the capacitors voltages or their corresponding passive circuits become unbalance. This can be corrected by modulating different voltages with the adapting modules instead of simply leaving all their semiconductors state fixed during the fault (e.g. in an open state). While the adapting modules passive circuits are bypassed, their capacitors are discharged through the resistors (e.g. semiconductors in a closed state).

A possible voltage control method would comprise the following steps for each proposed adapting module:

• measuring a characteristic voltage of the passive circuit, (such as the capacitor voltage or the whole circuit voltage);

• provide this measure to a regulator along with a reference for it;

• and modulating (by means of pulse-width-modulation) an output voltage according to the output of the regulator.

These regulators are independent from each other and may use the branch current value to self-adjust their parameters. The voltage of other nodes in the converter can also be used for feed forward control structure. The voltage reference for the adapting modules can be chosen so that the total voltage is greater than the maximum voltage the DC-fault may produce between the terminals of the branch. This control structure may be applied as soon as the fault is detected or after one or more modules capacitors have reached a particular voltage.

It is also possible to use the PWM (pulse-width-modulation) technic to impose a particular output voltage for the group of adapting modules of each branch while still maintaining balance among them. To do so, a regulator is used which receives a characteristic voltage (such as the capacitor voltage or the whole circuit voltage) of each adapting module along with the total output voltage reference. The regulator sorts the branch adapting modules in ascending or descending order of capacitor voltage and allocates the total output voltage reference among the ones which are most discharged. Finally each adapting module uses the PWM technic to produce the output voltage which the regulator chooses for it. It must be noted that due to the voltage that the current produces on the resistor of the passive circuit, the whole passive circuit voltage is different depending on whether the adapting module semiconductor is on or off. This must be taken into account when distributing the voltage modulation.

Advantageously, the possibility of modulating the voltage imposed by the branch provides the capability to control the fault current or the power which is being fed to the defective grid. In particular it is possible to reduce this current or power slowly and linearly if desired. The resulting cascade control scheme consists of two levels. The first level uses a regulator that receives the branch current or power and a reference for it, and returns the voltage that must be modulated by the branch. The second level allocates this voltage among the adapting modules as indicated in the previous paragraph. If the converter already comprises a regulator scheme to control its branch currents or the power transferred to the DC line, such regulator scheme can be used as the first level of the aforementioned cascade control scheme. During a DC fault, the output of this original current control scheme is provided to the cascade second level instead of being provided to the regular modules.

Brief description of the figures

• Figure 1 shows a one-arm converter. This arm joins the positive and negative terminal of a DC line (3 and 4) to an AC terminal through two branches. Multiphase converters may include several one-arm converters connected in parallel to the DC line positive and negative.

• Figure 2 shows the topology of a regular half-bridge module.

• Figure 3 shows a possible topology for the proposed adapting module.

• Figure 4 shows how the proposed adapting module is connected to the half- bridge modules in a branch.

• Figure 5 shows the topology of a typical full-bridge module.

• Figure 6 shows a possible way to control the capacitor voltage of one of the proposed adapting modules. Each adapting module would be controlled independently.

· Figure 7 shows a different possibility to control the capacitor voltage of the proposed adapting modules where all the modules of a branch are controlled together. Detailed description of the invention

In connection with the drawings, this section describes an embodiment of the invention, which is not intended to limit its scope.

Figure 1 shows a converter 1 with one arm. This arm is connected through two terminals 3 and 4 to a DC line and through another terminal 22 to an AC line. A branch 21 is formed between the AC terminal and each of the DC terminals. Each branch 21 includes a series of modules 2 and at least one inductance 5. Most of these modules are half-bridge modules 6 like the one shown in figure 2. The converter 1 can be realized with other types of regular modules such as the full-bridge module 16 shown in figure 5, although they would be more complex and expensive. Some of the modules of each branch 21 are the proposed adapting modules 11 with a topology similar to the one shown in figure 3.

The half-bridge modules 6 comprise a capacitor 7 and two controllable semiconductors 8 with their corresponding antiparallel diodes and which can be used to connect or disconnect the capacitor 7 in series with its terminals 9 and 10. The full-bridge modules 16 also comprise a capacitor 17, but they comprise four controllable semiconductors 18, instead of just two, which permit to connect the capacitor 17 with either polarity between its terminals 19 and 20. The proposed adapting module 11 comprises one controllable semiconductor 13, the corresponding antiparallel diode and a passive circuit. For this example, the passive circuit is an RC-series circuit 12 capable of supporting a short-circuit current due to DC line fault for a time several times greater than the time required for switching the semiconductors 8. Depending on the controllable semiconductor state, the RC-series circuit may be connected in series with the terminals 14 and 15 of the adapting module 11 or bypassed. When bypassed, the capacitor 12C, the resistor12R and the semiconductor 13 form a mesh that allows the capacitor energy to be discharged through the resistor. Other elements such as a varistor or a bypass mechanical contractor may be added to these adapting modules 11 .

The connection of the different modules 6, 11 can be seen in figure 4, where part of a branch 21 is shown. The positive terminal 9 of each half-bridge module 6 and the negative terminal 14 of each adapting module 11 are orientated to the positive terminal 3 of the DC connection, while the half-bridge modules negative terminals 10 and the proposed adapting modules positive terminals 15 are orientated to the negative DC connection terminal 4. Due to the full-bridge modules symmetry, their terminals 19 and 20 can be orientated either way.

During normal operation, the adapting modules controllable semiconductors 13 are saturated, so the passive circuits 12 are bypassed. The converter 1 is controlled with the regular modules, which may be a half-bridge module 6, or a full-bridge module 16.

Upon a fault in the DC-line, the voltage between terminals 3 and 4 falls, producing a high current l _B on the branches in the direction indicated in figure 4. When this occurs, all the regular modules 6, 16 are commanded to open all their controllable semiconductors. This way, the regular modules voltage will not favour the current. Depending on the current direction and on the module topology, their voltage may oppose the current. Even if the commutation was performed instantly, the AC terminal voltage will continue to supply energy to the inductances, increasing the branch current. To extinguish this current, the proposed modules semiconductors 13 are switched off. This way the passive circuits 12 are connected to the branch 21 . The current that circulates through the passive circuit 1 2 will produce a voltage which opposes it. Part of the energy stored in the inductances 5 is transferred to the capacitors, while the rest is dissipated in the resistors.

After the voltage of at least one of the adapting modules capacitor has reached certain value, the balance control is activated. A possible way to do so is to use a regulator 23 for each of these adapting modules as shown in figure 6. The regulators 23 receive the adapting module capacitor voltages (V _c ) and a reference for them (V _c Re , as well as the current that circulates through the branch 21 . Each regulator 23 returns an output voltage (V _mod ) for the adapting module 1 1 to modulate using PWM. Since the higher this output voltage is, the more the adapting module will charge and the less it will discharge, the regulator 23 will choose the necessary output for the capacitor voltage to reach its references. The references are chosen so that the maximum voltage the adapting modules can modulate is higher than the maximum voltage the branch will be subject to. For example, if there are six adapting modules, each reference will be higher than the sixth part of the maximum voltage between the DC terminal 3 or 4 and the AC terminal 22. A modulator 25, which may be implemented on the same physical device as the aforesaid regulator 23, receives the comparison value that corresponds to the voltage (V _mod ) which the adapting module is intended to modulate and produces the pulsed signal for the adapting module. For this example, the pulsed signal is produced by comparing a triangular signal with the comparison signal corresponding to regulator output (V _mod ).

Another possible way to balance the capacitors is to use the same regulator scheme the converter uses to control the power exchanged with the DC-Line. This power controller will provide the branch voltage references necessary in order to exchange a certain power reference with the DC-Line; in particular, it will attempt to lower the power the converter sends to the line linearly. Once the branch voltage references ( _B ranch Ref) have been determined, they are provided to controllers similar to the one shown in figure 7 and which can be implemented on a FPGA or on a DSP. The voltage provided by each branch is regulated by a different controller. The controller of each branch, in addition to the voltage reference for the branch, receives the branch current (branch) and the voltage of each of the adapting modules capacitor (V _C i , V _C2 ...). The capacitor voltages are corrected by adding the voltage that the branch current will produce on the resistors. Afterwards, the multivariable regulator 24 selects the voltage ( mocM , V _mod 2- - - ) that each adapting module 1 1 will modulate so that the branch voltage reference is reached and the adapting modules capacitor voltages tend to balance. In this example, the multivariable regulator 24 simply orders the adapting module capacitor voltages from higher to lower and distributes the branch voltage reference among the most discharged ones. To actually obtain the pulse width modulated signal, a modulator 25 can be used just like in the previous example. Both the described multivariable regulator 24 and the modulator 25 could be implemented on a DSP or an FPGA.

When the current that flows through the branch is low enough, the DC line is disconnected from the converter 1 . If the branch current becomes null due to the adapting modules capacitor voltage, it does not reverse because of the half-bridge modules voltage which would oppose it. Consequently, it is possible to lower the current as much as necessary providing the heat produced in the resistors can be dissipated fast enough.

Once the defective DC line has been disconnected, the controllable semiconductors of the adapting modules are closed again and the regular modules return to their modulation routine. The capacitors of the passive circuits 12C are discharged through their corresponding resistors 12R. Any service which does not require the DC line can continue to be supplied.

The voltage of the DC line is measured to detect when the fault has been cleared. When the line is operational again, the converter 1 can be connected back to it and continue to work normally.

Numerical references:

I Modular voltage converter.

2 Module (unspecified).

3 Positive DC voltage terminal.

4 Negative DC voltage terminal.

5 Inductance.

6 Half-bridge module.

7 Capacitor of a regular half-bridge module.

8 Controllable semiconductor of a regular half-bridge module.

9 First half-bridge module terminal.

10 Second half-bridge module terminal.

I I Adapting module.

12 Passive circuit.

12C Capacitor of an adapting module.

12R Resistor of an adapting module.

13 Controllable semiconductor of an adapting module.

14 First adapting module terminal.

15 Second adapting module terminal.

16 Full-bridge module.

17 Capacitor of a full-bridge module.

18 Controllable semiconductor of a full-bridge module.

19 First full-bridge module terminal.

20 Second full-bridge module terminal.

21 Branch.

22 AC terminal. Regulator.

Multivariable regulator Modulator.