CN109155598B

CN109155598B - HVDC converter system, control method thereof and HVDC system using HVDC converter system

Info

Publication number: CN109155598B
Application number: CN201680086011.3A
Authority: CN
Inventors: 马茨·安德森; 苑春明; 张利东; 杨晓波
Original assignee: ABB Grid Switzerland AG
Current assignee: Hitachi Energy Co ltd
Priority date: 2016-10-31
Filing date: 2016-10-31
Publication date: 2021-04-27
Anticipated expiration: 2036-10-31
Also published as: WO2018076329A1; CN109155598A

Abstract

The invention provides an HVDC converter system, a method thereof and an HVDC system using the converter system. The HVDC converter system comprises a plurality of VSCs (110, 111, 120, 121) coupled in series at their DC side, wherein two terminals of the series coupling are configured to be coupled with an HVDC network; a plurality of bypass breakers (112, 113, 122, 123), each coupled in parallel with the DC side of a respective one of the plurality of VSCs (110, 111, 120, 121), and closed if its respective VSC is latched; and a controller (15) adapted to control the current flowing through the closed bypass breaker to zero and subsequently to control the closed bypass breaker to open. By controlling the bypass breaker associated with the VSC to first reach zero current and then to open the bypass breaker and unlock the VSC, the bypass breaker can be opened in a reliable manner independent of the zero crossing of the bypass breaker that would naturally occur, otherwise, by using the converter system built by the LCC. Thus, a latched VSC of a converter system for a converter station of an HVDC transmission system can reliably be put into operation while another VSC is still in operation.

Description

HVDC converter system, control method thereof and HVDC system using HVDC converter system

Technical Field

The present invention relates to HVDC systems and more particularly to HVDC converters arranged for operation in series, e.g. for multiplication of voltage.

Background

High Voltage Direct Current (HVDC) transmission system comprising at each end of an HVDC network converter stations for connecting said network to an Alternating Current (AC) system, each of said stations comprising a series connection of at least two converters, the Direct Current (DC) sides of said converters being connected on the one hand to poles of said network at high potential and on the other hand to a neutral bus at zero potential by earth, a first of said converter stations being adapted to operate as a rectifier and the other (second) as an inverter, each converter having a bypass breaker coupled in parallel in a current path forming a bypass of said converter when said breaker is closed, each converter station comprising an arrangement adapted to control the DC current through said poles, and means for each converter of said stations to increase the voltage between said neutral bus and said poles by initiating operation of the converter and to increase the voltage between said neutral bus and said poles when the converter is blocked The apparatus reduces the voltage between said neutral bus and said poles by increasing the voltage, or when a converter is activated, by stopping the operation of said converter, and reduces the power transmitted between said stations by reducing the voltage, and a method for controlling such a transmission system.

The converter comprises a plurality of current valves of any known configuration, for example in a 12-pulse bridge configuration. The converter may be a line commutated current source converter (LCC) in which switching elements, such as thyristors, are turned off at zero crossings of the AC current in said AC system.

When the voltage obtained between the neutral bus and the pole is too high to be obtained by only one converter, two or more converters are usually connected in series. If all converters of a converter station fail and cause the power transmitted through the system to drop to zero, large disturbances may be caused with very serious consequences for the AC system connected to said HVDC transmission system. This is the main reason for arranging said bypass current path for each converter so that a converter which is operating in error can thus be bypassed and possibly disconnected for maintenance, while the converter station can be operated by controlling the other converters of the station. It is therefore important to be able to stop such converters in a way that it does not cause interference in the transmission system. The same applies to the procedure for starting the converter of such a system for raising the voltage between the neutral bus and said poles and by that raising the power transmitted between said stations.

Putting the converter into operation is rather complicated, as it should be both fast and safe. The critical issue is to open the bypass breaker in a safe manner. For LCC HVDC transmission systems, there are naturally a large number of harmonics in the DC voltage, driving harmonic currents. Due to harmonic currents, zero crossings naturally occur in the current through the bypass breaker, so that a standard AC circuit breaker can be used as a bypass breaker.

However, the converter may also be a forced commutated Voltage Source Converter (VSC), wherein said switching elements are semiconducting devices (e.g. IGBTs or IGCTs) forced to turn off according to a Pulse Width Modulation (PWM) mode. For example, with the MMC VSC HVDC transmission system, the DC voltage is naturally free of harmonics. Thus, opening a bypass breaker is practically infeasible because the current through it does not cross zero. There is a significant amount of internal arcing inside the bypass breaker until it is destroyed.

Disclosure of Invention

According to an aspect of the invention, there is provided an HVDC converter system comprising: a plurality of VSCs coupled in series on their DC side, wherein two terminals of the series coupling are configured to be coupled with the HVDC network; a plurality of bypass breakers, each coupled in parallel with the DC side of a respective one of the plurality of VSCs and closed with its respective VSC latched; and a controller adapted to control the current flowing through the closed bypass breaker to zero and subsequently to control the closed bypass breaker to open.

According to another aspect of the invention, there is provided an HVDC system using an HVDC converter system.

According to another aspect of the invention, there is provided a method for controlling an HVDC converter system, wherein: an HVDC converter system comprising: a plurality of VSCs coupled in series on their DC side, wherein two terminals of the series connection are configured to be coupled with the HVDC network; and a plurality of bypass circuit breakers, each coupled in parallel with the DC side of a respective one of the plurality of VSCs and closed upon latching of its respective VSC; the method comprises the following steps: (a) controlling the current through the closed bypass breaker to zero; and (b) controlling the closed bypass breaker to open after step (a).

By controlling the bypass breaker associated with the VSC to first reach zero current and then open the bypass breaker and unlock the VSC, the bypass breaker can be opened in a reliable manner without relying on the zero crossing of the bypass breaker that would naturally occur, otherwise, by using the converter system built by the LCC. Thus, a latched VSC for a converter system of a converter station of an HVDC transmission system can reliably be put into operation while another VSC is still in operation.

Preferably, the controller is further adapted to control at least one of the plurality of VSCs to generate a harmonic voltage on its DC side so as to inject a zero crossing harmonic current flowing through the closed bypass breaker and to control the closed bypass breaker to open at the zero crossing of the harmonic current. Preferably, step (a) further comprises: controlling at least one of the plurality of VSCs to generate a harmonic voltage on its DC side for injecting a zero-crossing harmonic current flowing through the closed bypass circuit breaker; and step (b) further comprises: controlling the closed bypass breaker to open at the harmonic current zero crossing.

By controlling the VSC to inject said harmonic current into its associated bypass breaker, the bypass breaker can be opened in a reliable manner without relying on naturally occurring harmonic currents, which would otherwise be obtained by using a converter system built from LCCs. Thus, a latched VSC for a converter system of a converter station of an HVDC transmission system can reliably be put into operation while another VSC is still in operation.

Preferably, the HVDC converter system further comprises: a first current measuring device for measuring a first value of the current through the HVDC network; and second current measuring means for measuring a second value of the current flowing through the latched VSC; wherein: the controller is further adapted to calculate a difference between the first value and the second value, the difference representing a third value of the harmonic current flowing through the closed bypass circuit breaker to determine a zero crossing of the harmonic current.

Preferably, the latched VSC is operated under the control of the controller as a VSC generating harmonic voltages. This makes it possible to reuse the capacity of the VSC that has not yet been operated, reduce the workload in operation, and simplify the control thereof.

Preferably, the unlocked VSC is operated under control of the controller as a VSC generating a harmonic voltage and having on its DC side a combination of the harmonic voltage and its operating DC voltage; alternatively, both the latched VSC and the unlatched VSC operate as VSCs that generate harmonic voltages under the control of the controller. In some cases, the VSC to be switched in is not allowed to be unlocked to generate harmonic voltages, or is not operable to generate harmonic voltages large enough to inject a minimum harmonic current into its associated bypass breaker, which can also be opened in a reliable manner by having other VSCs (other than to be switched in) generate harmonic voltages in their place or in combination therewith.

Preferably, the harmonic voltage is an nth harmonic, n being an odd number; for example, the harmonic voltage may be 3 rd harmonic or 5 th harmonic. Otherwise, even harmonics may have a negative impact on the AC system and the converter transformer connected to the VSC.

Preferably, the HVDC converter system further comprises: a power diode interposed between said bypass breaker and its corresponding VSC-blocking short circuit current loop involving closing of the freewheeling diodes of the forced turn-off semiconductor devices of the bypass breaker and its corresponding VSC. This makes it possible to prevent a short circuit that might otherwise occur during a period when one of the VSCs is taken out at the receiving end of the HVDC transmission system.

Preferably, the plurality of VSC forced turn-off based semiconductor devices are configured to operate as a receiving end of the HVDC network; and the controller is further adapted to control to block the transmitting side of the HVDC network for a predetermined period of time. Thus, the current followed by the closing bypass breaker is controlled to reach zero and subsequently the closing bypass breaker is controlled to open.

Drawings

The subject matter of the invention will be explained in more detail hereinafter with reference to preferred exemplary embodiments shown in the drawings, in which:

fig. 1 shows an HVDC transmission system according to a first embodiment of the invention;

FIG. 2 shows a block diagram of a local controller according to a first embodiment of the invention; and

fig. 3 shows an HVDC transmission system according to a second embodiment of the invention.

The reference symbols used in the drawings and their meanings are listed in summary form in the list of reference symbols. In principle, identical components have the same reference numerals in the figures.

Detailed Description

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Further, it is noted that, in this application, the word "may" is used in a permissive sense (i.e., having the potential to, being able to), rather than the mandatory sense (i.e., must). The term "comprising" and its derivatives mean "including but not limited to". The term "connected" means "directly or indirectly connected", and the term "coupled" means "directly or indirectly connected".

Fig. 1 shows an HVDC transmission system according to a first embodiment of the invention. As shown in fig. 1, the HVDC transmission system 1 is schematically shown to comprise a first converter station 11 and a second converter station 12 at each end of an HVDC network 10 for connecting said HVDC network 10 to a first AC system 13 and a second AC system 14. The first AC system 13 is assumed to be a power generation system in the form of any type of power plant with generators of electricity, while the second AC system 14 is assumed to be a consumer system or network connected to electrical power consumers (e.g. industries and communities). Thus, the first converter station 11 is adapted to act as a rectifierThe machine operates and the other, second converter station 12, operates as an inverter. The first converter station 11 comprises an HVDC converter system having a plurality of VSCs (voltage source converters) coupled in series on its DC side, wherein both terminals of the series coupling are configured to be coupled with the HVDC network 10; for example, as shown in fig. 1, the converter system of the first converter station 11 has two VSCs 110, 111 coupled in series with each other on their DC side, and the converter system of the first converter station 11 has two terminals T coupled in series_p11，T_n11Both of which are connected on the one hand to the positive pole of the high potential of the HVDC network 10 and on the other hand to the zero potential on the neutral bus of the HVDC network 10 through ground. Each of the converters 110, 111 comprises a number of converter valves of known configuration, for example a 12-pulse bridge configuration or an MMC configuration. These valves are formed by a plurality of power semiconductor devices connected in series for holding a high voltage in common in their latched state. Similarly, the converter system of the second converter station 12 has two

VSCs

120, 121 coupled in series with each other on their DC side, and the second converter station 12 has two terminals T coupled in series_p12，T_n12Connected on the one hand to the positive pole of the HVDC network 10 at high potential and on the other hand to zero potential on the neutral bus of the HVDC network 10 through ground. Each of the

converters

120, 121 comprises a number of converter valves of known configuration, for example a 12-pulse bridge configuration or an MMC configuration.

The HVDC transmission system 1 further comprises a plurality of bypass breakers, each coupled in parallel with the DC side of a respective one of the plurality of VSCs, and closed if its respective VSC is latched. For example, as shown in fig. 1, each of the

VSCs

110, 111, 120, 121 has a current path P₁₁₀、P₁₁₁、P₁₂₀、P₁₂₁And a

bypass breaker

112, 113, 122, 123 connected in parallel therewith, the current path P being such that when said

bypass breaker

112, 113, 122, 123 is closed₁₁₀、P₁₁₁、P₁₂₀、P₁₂₁A bypass of the

VSC

110, 111, 120, 121 is formed.

The HVDC transmission system 1 further comprises a controller 15 adapted to control the current through the closed bypass breaker to zero and subsequently to control the closed bypass breaker to open. For example, the controller 15 includes a master controller 150 and

local controllers

1510, 1511, 1520, 1521, each controller being operatively associated with a

respective VSC

110, 111, 120, 121. Under their control, the VSC associated with them begins to operate when latched, and stops its operation when the VSC is unlatched.

It is assumed that one VSC of the first converter station 11, i.e. the VSC 110, has been latched for maintenance or the like, and that the current path P₁₁₀Having turned on by closing the bypass breaker 112 and the VSC 110 is put into operation, the main controller 15 can send an unlock instruction to the local controller 1510 associated with the VSC 100. Further assume that the

other VSCs

111, 120, 121 are already running and their associated

bypass breakers

113, 122, 123 are opened, blocking the current paths P111, P120, P121.

Fig. 2 shows a block diagram of a local controller according to a first embodiment of the invention. As shown in fig. 2, the local controller comprises a first input terminal 20 for receiving the voltage reference value, a second input terminal 21 for receiving the harmonic voltage reference value, an adder 22 for adding the inputs from the two

input terminals

20, 21, and an output terminal 23 for outputting the result of the addition to a PWM generator, which in turn generates a PWM signal to the associated VSC, which approximates the addition of the voltage reference value and the harmonic reference value. Upon receiving an unlock instruction from the main controller 15, the local controller 1510 for the VSC 110 then takes over control of the VSC 110 by unlocking it, generating a harmonic voltage on its DC side by starting to control it, such that via the conduction current path P₁₁₀Zero-crossing harmonic currents flowing through the closed bypass breaker 112 are injected. To generate harmonic voltages on the DC side of the associated VSC, the voltage reference value may be set to zero and the harmonic voltage reference value to the nth harmonic. By switching the voltage to the associated VSC with a suitable duty cycle, the output of the PWM generator will approach the voltage at the harmonic voltage reference value. The harmonic voltages generated on the DC side of the VSC 110 are applied to an associated bypass breaker 112, which is connected to the DC side of the VSCHas a substantially zero DC component so that harmonic currents flow through the bypass breaker 112 and have zero crossings. That is, the closed bypass breaker is controlled to open at the zero crossing of the harmonic current. After confirming that the current in the bypass breaker 112 has a zero crossing so that the bypass breaker can be opened, an open command to the bypass breaker 112 is given by the local controller 1510. To the right part of fig. 2, the waveform of the current flowing through the bypass breaker according to the first embodiment is shown. As shown in fig. 2, the vertical axis represents the bypass breaker current I_by-passAnd the horizontal axis represents time. From t₀To t₁The time period in which the VSC 110 is latched with its associated bypass breaker 112 closed, and the bypass breaker is conducting DC current I_DC. From t₁To t₂The period of time in which the VSC 110 applies a harmonic current on its DC side, injecting the harmonic current into the bypass breaker 112, the bypass breaker current with harmonics being derived from the direct current I_DCBiased towards zero. From t₂To t₃In time periods, the bypass breaker current oscillates around zero; at a point in time t₃Which reaches one of the zero crossings, the bypass breaker 112 is opened and the VSC 110 is switched in. At a point in time t₃Thereafter, the bypass breaker 112 remains open to conduct substantially zero current.

By controlling the VSC to inject harmonic currents into its associated bypass breaker, the bypass breaker can be opened in a reliable manner without relying on harmonic currents that may naturally occur, otherwise through the use of converter systems built from LCCs. Thus, a latched VSC for a converter system of a converter station of an HVDC transmission system can reliably be put into operation while another VSC is still in operation.

After the bypass breaker acknowledges the open indication, the reference voltage of the local controller 1510 is changed to normal operation with its harmonic reference set to zero. Thus, the PWM generator generates a PWM from the reference voltage reference and adjusts the delay angle of the VSC accordingly. The VSC will continue to operate normally after successfully opening its associated bypass breaker. An HVDC converter system may comprise: a first current measuring device 16 for measuring a first value of the current through the HVDC network; and a second current measuring device 1610, 1611, 1620, 1621 for measuring a second value of the current flowing through the latched VSC 110-, 111, 120, 121.

Following the above assumptions, one VSC of the first converter station 11, i.e. the VSC 110, has been latched and the current path P has been latched₁₁₀Having been turned on by closing the bypass breaker 112, the associated local controller 1510 may calculate a difference between the first value and the second value, the difference being indicative of a third value of the harmonic current flowing through the bypass breaker 112 to determine a zero crossing of the harmonic current.

As described above, the first embodiment of the present invention is described by way of example, in which the VSC that has been latched operates as a VSC that generates harmonic voltages under the control of the controller.

Alternatively, the unlocked VSC can be operated under the control of the controller as a VSC generating harmonic voltages, and the harmonic voltages combined with the operating direct voltage on its DC side. The frequency of the "zero crossing generator" oscillation can be chosen according to practical needs, for example, simple harmonic frequencies are fully possible. Examples will be described assuming the following cases: one VSC of the first converter station 11, i.e. VSC 110, has been latched and the other VSC, i.e. VSC 111, has been operated (has been unlatched) and DC current has flowed through current path P₁₁₀Through the closed bypass breaker 112 and the operating VSC 111. When the VSC 111 is controlled by its local controller 1511 to generate a harmonic voltage on its DC side, a harmonic voltage having a polarity opposite to that of the harmonic voltage on the DC side of the VSC 112 will be generated on the closed bypass breaker 112, since the DC voltage on the HVDC network remains substantially constant. Thus, harmonic currents will be injected into the current path P through the closed bypass breaker 112₁₁₀And the closed bypass breaker 112 may open at its current zero crossing. It will be appreciated by those skilled in the art that both the latched VSC and the unlatched VSC can be operated as VSCs that generate harmonic voltages under the control of the controller.

In some cases, the VSC to be switched in is not allowed to be unlocked to generate harmonic voltages, or is not operable to generate harmonic voltages large enough to inject a minimum harmonic current into its associated bypass breaker, which can also be opened in a reliable manner by other VSCs (in addition to the VSC to be switched in) generating harmonic voltages in its place or in combination with it.

Preferably, the harmonic voltage is an nth harmonic, n being an odd number; for example, the harmonic voltage may be 3 rd harmonic or 5 th harmonic. Otherwise, even harmonics can have a negative impact on the AC system and the converter transformer connected to the VSC.

Fig. 3 shows an HVDC transmission system according to a second embodiment of the invention. In order to avoid redundancy and to keep brevity, the second embodiment is described based on the description of the first embodiment.

It will be appreciated by those skilled in the art that VSCs (voltage source converters) consist of semiconductor devices that are forced to turn off, such as IGBTs and IGCTs, can be controlled to turn on and off, providing a second degree of freedom. In such an inverter, the polarity of the DC voltage is generally fixed, and the DC voltage smoothed by the large capacitance may be regarded as constant. In order to allow reverse current to flow, a forced-off semiconductor device is usually provided in parallel with an additional diode (freewheeling diode) in its components to conduct current in the opposite direction. There are several different configurations of VSCs, including, for example, two-level VSCs, three-level VSCs, and modular multilevel VSCs (mmcs).

At the receiving end of the HVDC transmission system, the

VSCs

120, 121 of the second converter station 12 are operated in inverter mode; whereas on the transmitting side the VSCs 110, 111 of the first converter station 11 are operated in rectifier mode.

Assuming that the

VSC

110, 111, 120, 121 has been operated, its associated

bypass breaker

112, 113, 122, 123 is opened, blocking the current path P₁₁₀、P₁₁₁、P₁₂₀、P₁₂₁. In addition, one VSC of the second converter station 12, i.e. the VSC 120, will be latched for maintenance and its open bypass breaker 122 will be closed, allowing conduction P₁₂₀So that the HVDC transmission system will be able to operate with the remaining VSCs 110, 111, 121. Here, the VSC 120 adopts a configuration of a two-stage converter as an example for describing the present invention. The two-stage VSC 120 has a six-pulse bridge with each branch havingThere are series-coupled forced-off

semiconductor devices

1200, 1201, 1202, 1203, 1204, 1205, each having a freewheeling diode D0, D1, D2, D3, D4, D5 in anti-parallel, and a DC smoothing capacitor C disposed on the DC side thereof.

In this case, there will be a short circuit between the positive and negative terminals of the DC side of the VSC 120 involving the freewheeling diodes D0, D1, D2, D3, D4, D5 of the VSC 120 and the closed bypass breaker 122. This results in excessive current being limited only by the relatively small resistance of the network and may result in circuit damage, overheating, fire or explosion. Arrow P_scIndicating a short circuit path.

To prevent a short circuit during the extraction of one VSC, e.g. VSC 120, at the receiving end of the HVDC transmission system, the converter systems of the second converter station 12 each further comprise

power diodes

30, 31. As shown in fig. 3, the power diode 30 is inserted into the connection between the DC side of the VSC 120 and its associated bypass breaker 122, and the forward direction of the power diode 30 is opposite to the forward direction of the freewheeling diode of the VSC 120; the power diode 31 is inserted in the connection between the DC side of the VSC 123 and its associated bypass breaker 123, and the power diode 31 is opposite to the forward direction of the freewheeling diode of the VSC 121. In summary, the power diodes are inserted between the bypass breaker and its corresponding VSC blocking short circuit current loop involving the closed bypass breaker and the freewheeling diode of the forced turn-off semiconductor arrangement of its corresponding VSC, which may be represented as power diodes placed on the DC line for DC fault handling.

Thus, the short-circuit path P_scWill be blocked by the power diodes 30 because during the extraction (tripping-out) of the VSC 120 their directions are opposite to each other, while the other VSCs 110, 111, 121 can still be operated, so the HVDC transmission system will not be suspended. Furthermore, due to the fact that most HVDC transmissions are transmitted in one direction only, DC fault handling in hybrid HVDC systems can be done using large diodes placed on the DC transmission line. Since the

power diodes

30, 31 can also handle DC faults, the use used in the transmission lines of conventional HVDC transmission systems can be removedHigh power diodes for DC fault handling.

Assuming that the VSC 120 has been latched and will be switched in again (taken in), a zero crossing of the current cannot be achieved since the power diode 30 will completely block the harmonic current injected according to the first embodiment, making the opening of the associated bypass breaker 122 infeasible. In contrast, according to the second embodiment, upon receiving an unlock (de-blocking) instruction from the main controller 15, the

local controllers

1510, 1511 for the VSCs 110, 111 of the first converter station 11 will latch their VSCs 110, 111 on the transmit side for a predetermined period of time, e.g. a few hundred milliseconds, during which the DC current flowing through the HVDC network 10 drops to substantially zero; the local controller 1520 for the VSC 120 then takes over control of the VSC 120 by opening the associated bypass breaker 122, rather than generating harmonic voltages on its DC side as disclosed in the first embodiment. That is, the current flowing through the closed bypass breaker is controlled to zero and the subsequently closed bypass breaker is controlled to open.

By controlling the bypass breaker associated with the VSC to be switched in to first reach zero current and then to open the bypass breaker and unlock the VSC, the bypass breaker can be opened in a reliable manner without relying on the zero crossing of the bypass break that naturally occurs, otherwise, by using a converter system built from LCCs. Thus, a latched VSC of a converter system for a converter station of an HVDC transmission system can reliably be put into operation while another VSC is still in operation.

Although the present invention has been described based on some preferred embodiments, those skilled in the art should understand that those embodiments should not limit the scope of the present invention in any way. Any variations and modifications of the embodiments herein described should be considered within the purview of one of ordinary skill in the art and the understanding of those skilled in the art without departing from the spirit and intended scope of the invention as defined by the appended claims.

Claims

1. An HVDC converter system comprising:

a plurality of VSCs coupled in series on their DC side, wherein two terminals of the series coupling are configured to be coupled with the HVDC network;

a plurality of bypass breakers, each coupled in parallel with the DC side of a respective one of the plurality of VSCs and closed with its respective VSC latched; and

a controller adapted to control the current flowing through a closed bypass breaker to zero and subsequently to control the closed bypass breaker to open;

wherein:

the controller is further adapted to control at least one of the plurality of VSCs to generate a harmonic voltage on its DC side for injecting a zero crossing harmonic current flowing through the closed bypass breaker and to control the closed bypass breaker to open at a zero crossing of the harmonic current.

2. The HVDC converter system of claim 1, further comprising:

a first current measuring device for measuring a first value of the current through the HVDC network; and

a second current measuring means for measuring a second value of the current flowing through the latched VSC;

wherein:

the controller is further adapted to calculate a difference between the first value and the second value, the difference representing a third value of the harmonic current flowing through the closed bypass circuit breaker to determine a zero crossing of the harmonic current.

3. The HVDC converter system of claim 1 or 2, wherein: the latched VSC generates the harmonic voltage under the control of the controller.

4. The HVDC converter system of claim 1 or 2, wherein: the unlocked VSC is operated under the control of the controller as the VSC generating the harmonic voltage and combines the harmonic voltage with its operating DC voltage on its DC side.

5. The HVDC converter system of claim 1 or 2, wherein: the latched VSC and the unlatched VSC both operate as the VSC that generates the harmonic voltages under the control of the controller.

6. The HVDC converter system of claim 1 or 2, wherein: the harmonic voltage is an nth harmonic and n is an odd number.

7. The HVDC converter system of claim 1, wherein:

the VSC are based on semiconductor devices with forced turn-off and are configured to operate as a receiving end of the HVDC network; and

the controller is further adapted to control to latch a transmitting end of the HVDC network for a predetermined time.

8. The HVDC converter system of claim 7, further comprising:

a power diode interposed between the bypass breaker and its corresponding VSC-blocking short circuit current loop involving closing of the freewheeling diodes of the forced turn-off semiconductors of the bypass breaker and its corresponding VSC.

9. An HVDC system using the HVDC converter system of any of the preceding claims.

10. A method for controlling an HVDC converter system, wherein:

the HVDC converter system comprises:

a plurality of VSCs coupled in series on their DC side, wherein two terminals of the series coupling are configured to be coupled with the HVDC network; and

a plurality of bypass breakers, each coupled in parallel with a DC side of a respective one of the plurality of VSCs and closed with its respective VSC latched;

the method comprises the following steps:

(a) controlling the current through the closed bypass breaker to reach zero; and

(b) controlling the closed bypass breaker to open after step (a);

wherein:

step (a) further comprises:

controlling at least one of the plurality of VSCs to generate a harmonic voltage on its DC side such that a zero-crossing harmonic current flowing through the closed bypass breaker is injected; and

step (b) further comprises:

controlling the closed bypass breaker to open at the harmonic current zero crossing.

11. The method of claim 10, wherein:

the step (a) further comprises:

controlling to block a transmitting end of the HVDC network for a predetermined time.

12. The method of claim 11, further comprising:

blocking the short circuit current loop, the freewheeling diode of the forced turn-off semiconductor device involving the closed bypass breaker and its corresponding VSC.