WO2018076329A1

WO2018076329A1 - Hvdc converter system and control method therefor and hvdc system using the same

Info

Publication number: WO2018076329A1
Application number: PCT/CN2016/103976
Authority: WO
Inventors: Mats Andersson; Chunming YUAN; Lidong ZHANG; Xiaobo Yang
Original assignee: Abb Schweiz Ag
Priority date: 2016-10-31
Filing date: 2016-10-31
Publication date: 2018-05-03
Also published as: CN109155598A; CN109155598B

Abstract

It provides an HVDC converter system, a method therefor and an HVDC system using the same. The HVDC converter system includes a plurality of VSCs (110,111,120,121) being coupled at their DC sides in series, wherein two terminals of the series connection are configured to be coupled with HVDC network; a plurality of by-pass breakers (112,113,122,123), each being in parallel coupled with the DC side of the corresponding one of the plurality of VSCs (110,111,120,121) and being closed where its corresponding VSC is blocked; and a controller (15), being adapted to control current following through the closed by-pass breaker to reach zero and subsequently control the closed by-pass breaker to open. By controlling the by-pass breaker associated with the VSC to be taken in to reach zero current in the first place and then opening the by-pass breaker and deblocking the VSC, the by-pass breaker can be opened in a reliable way independent of zero-crossing of the by-pass breaker naturally would have generated, otherwise, by using converter system built up of LCCs. Therefore, a blocked VSC of converter system of a converter station for HVDC transmission system can be reliably taken into operation, while the other VSC is still operating.

Description

HVDC CONVERTER SYSTEM AND CONTROL METHOD THEREFOR AND HVDC SYSTEM USING THE SAME

Technical Field

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

Background Art

HVDC (High Voltage Direct Current) transmission system comprises at each end of an HVDC network a converter station for connecting said network to an AC system, each said station comprising a series connection of at least two converters having a DC side thereof connected on one hand to a pole of said network on high potential and on the other to a neutral bus on zero potential by being earthed, a first of said converter stations being adapted to operate as rectifier and the other, second as inverter, each converter having a by-pass breaker connected in parallel therewith in a current path forming a by-pass of said converter when said breaker is closed, each converter station comprising an arrangement adapted to control the DC current through said pole and for each converter of the station a device for controlling that converter and the current therethrough for either increasing the voltage between said neutral bus and said pole and by that the power transmitted between said stations by starting operation of the converter when being blocked or reducing the voltage between said neutral bus and said pole and by that the power transmitted between said stations by stopping operation of the converter when being active, as well as a method for controlling such a transmission system.

The converters include a number of current valves in any known configuration, for instance in a 12 pulse-bridge configuration. The converters may be line commutated Current Source Converters (LCC) , in which the switching elements, such as thyristors, are turned off at zero crossing of the AC current in said AC system.

Two or more converters are normally connected in series when the voltage to be obtained between said neutral bus and said pole is too high for being possible to obtain through only one converter. It may cause large disturbances with very severe consequences on AC systems connected to a said HVDC transmission system if all the converters of a converter station and by that a converter station would fail and by that the power transmitted through the system falls to zero. This is the main reason for arranging said by-pass current path for each converter, so that an incorrectly operating converter may be by-passed and possibly disconnected for maintenance while the converter station may be operated by controlling the other converters of the station. It is then important to be able to stop such a converter in a way causing no disturbances in the transmission system. The same is applicable to a procedure for starting a converter of such a system for raising the voltage between the neutral bus and said pole and by that the power transmitted between said stations.

Taking a converter into operation is fairly complicated, since it should both be fast and safe. The key issue is opening the by-pass breaker in a safe way. For LCC HVDC transmission system, there will naturally be a high amount of harmonics in the DC voltage, which in turn drives a harmonic current. Due to the harmonic current, there will naturally be zero crossings in the current going through the by-pass breaker, thus a standard AC circuit breaker can be used as the by-pass breaker.

However, the converters may also be forced commutated Voltage Source Converters (VSCs) , in which said switching elements are force turned-off semiconductor devices according to a Pulse Width Modulation (PWM) pattern, such as IGBT or IGCT. For example, with MMC VSC HVDC transmission system, the DC voltage is naturally harmonic free. Therefore opening a by-pass breaker is not practically feasible, since there would be no zero crossings of the current going through it. There would be extensive internal arcing inside the by-pass breaker, until it would be destroyed.

Brief Summary of the Invention

According to an aspect of present invention, it provides an HVDC converter system including: a plurality of VSCs being coupled at their DC sides in series, wherein two terminals of the series connection are configured to be coupled with HVDC network； a plurality of by-pass breakers, each being in parallel coupled with the DC side of the corresponding one of the plurality of VSCs and being closed where its corresponding VSC is blocked； and a controller, being adapted to control current following through the closed by-pass breaker to reach zero and subsequently control the closed by-pass breaker to open.

According to another aspect of present invention, it provides a HVDC system using the HVDC converter system.

According to another aspect of present invention, it provides a method for controlling HVDC converter system, wherein: the HVDC converter system includes: a plurality of VSCs being coupled at their DC sides in series, wherein two terminals of the series connection are configured to be coupled with HVDC network； and a plurality of by-pass breakers, each being in parallel coupled with the DC side of the corresponding one of the plurality of VSCs and being closed where its corresponding VSC is blocked； the method including: (a) controlling current following through the closed by-pass breaker to reach zero； and (b) controlling the closed by-pass breaker to open subsequent to the step (a) .

By controlling the by-pass breaker associated with the VSC to be taken in to reach zero current in the first place and then opening the by-pass breaker and deblocking the VSC, the by-pass breaker can be opened in a reliable way independent of zero-crossing of the by-pass breaker naturally would have generated, otherwise, by using converter system built up of LCCs. Therefore, a blocked VSC of converter system of a converter station for HVDC transmission system can be reliably taken into operation, while the other VSC is still operating.

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

By controlling the VSC to inject the harmonic current into its associated by-pass breaker, the by-pass breaker can be opened in a reliable way independent of harmonic current naturally would have generated, otherwise, by using converter system built up of LCCs. Therefore, a blocked VSC of converter system of a converter station for HVDC transmission system can be reliably taken into operation, while the other VSC is still operating.

Preferably, the HVDC converter system further includes: a first current measurement device, for measuring a first value of current flowing through the HVDC network； and a second current measurement device, for measuring a second value of current flowing through the blocked VSC； wherein: the controller is further adapted for calculating a difference between the first value and the second value indicating a third value of the harmonic current flowing through the closed by-pass breaker so as to determine the zero-crossing of the harmonic current.

Preferably, the blocked VSC operates as the VSC generating the harmonic voltage under the control of the controller. This makes it possible to reuse the capacity of the VSC which has not be in operation, reducing work load of those in operation and simplifying the control therefor.

Preferably, the de-blocked VSC operates as the VSC generating the harmonic voltage under the control of the controller, with a combination of the harmonic voltage and its operating DC voltage at its DC side； as an alternative, both of the blocked VSC and the de-blocked VSC operate as the VSC generating the harmonic voltage under the control of the controller. In some scenario where the VSC to be taken in is not allowed to be de-blocked to generate the harmonic voltage or cannot operate to generate the harmonic voltage in sufficient magnitude for injection minimum harmonic current to its associated by-pass breaker, by having other VSC, besides the to-be taken in, generate the harmonic voltage in its replacement or in combination of it, the by-pass breaker can be opened in a reliable way, as well.

Preferably, the harmonic voltage is of n-th harmonic, n is odd number； for example, the harmonic voltage could be 3^rd or harmonic or 5^th. Otherwise, even number of harmonic would bring negative effect on the AC system and converter transformer connected with the VSC.

Preferably, the HVDC converter system further includes: a power diode, being inserted between the by-pass breaker and its corresponding VSC blocking short current loop involving the closed by-pass breaker and the freewheeling diodes of the force turned-off semiconductor device of its corresponding VSC. This makes it possible for preventing a short circuit that may, otherwise, occur in the period of taking-outing one of the VSC on the receiving end of the HVDC transmission system.

Preferably, the plurality of VSC are based on force turned-off semiconductor device, being configured to operate as receiving end of the HVDC network； and the controller is further adapted for controlling to block sending end of the HVDC network for a predetermined period of time. Therefore, the current following through the closed by-pass breaker is controlled to reach zero and subsequently the closed by-pass breaker is controlled to open.

Brief Description of the Drawings

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the drawings, in which:

Figure 1 shows an HVDC transmission system according to a first embodiment of the present invention；

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

Figure 3 shows an HVDC transmission system according to a second embodiment of the present 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 parts are provided with the same reference symbols in the figures.

Preferred Embodiments of the Invention

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. Furthermore, note that the word "may" is used throughout this application in a permissive sense (i.e., having the potential to, being able to) , not a mandatory sense (i.e., must) . " The term "include" , and derivations thereof, mean "including, but not limited to" . The term "connected" means "directly or indirectly connected" , and the term "coupled" means "directly or indirectly connected" .

Figure 1 shows an HVDC transmission system according to a first embodiment of the present invention. As shown in figure 1, the HVDC transmission system 1 includes at each end of the HVDC network 10 a first converter station 11 and a second converter station 12 for connecting said HVDC network 10 to a first AC system 13 and a second AC system 14 schematically indicated. The first AC system 13 is assumed to be a generating system in the form of any type of power plant with generators of electricity, whereas the second AC system 14 is assumed to be a consuming system or network connecting to consumers of electric power, such as industries and communities. Thus, the first converter station 11 is adapted to operate as rectifier and the other, the second converter station 12 as inverter. the first converter station 11 comprises an HVDC converter system having a a plurality of VSCs being coupled at their DC sides in series, wherein two terminals of the series connection are configured to be coupled with HVDC network 10； for example as shown in figure 1, the converter system of the first converter station 11 has two VSCs 110, 111 are series connected with each other at their DC sides, and have the two terminals T_p11, T_n11 of the series connection connected on one hand to a pole of positive polarity of the HVDC network 10 on high potential and on the other to a neutral bus of the HVDC network 10 on zero potential by being earthed. Each converter 110, 111 includes a number of converter valves in any known configuration, for instance in a 12-pulse bridge configuration or MMC configuration. These valves are formed by a plurality of power semiconductor devices connected in series for together holding a high voltage in the blocking state thereof. Similarly, the converter system of the second converter station 12 has two

VSCs

120, 121 are series connected with each other at their DC sides, and have the two terminals T_p12, T_n12 of the series connection connected on one hand to the pole of positive polarity of the HVDC network 10 on high potential and on the other to the neutral bus of the HVDC network 10 on zero potential by being earthed. Each

converter

120, 121 includes a number of converter valves in any known configuration, for instance in a 12-pulse bridge configuration or MMC configuration.

The HVDC transmission system 1 further includes a plurality of by-pass breakers, each being in parallel coupled with the DC side of the corresponding one of the plurality of VSCs and being closed where its corresponding VSC is blocked. For example as shown in figure 1, each

VSC

110, 111, 120, 121 has a by-

pass breaker

112, 113, 122, 123 connected in parallel therewith in a current path P₁₁₀, P₁₁₁, P₁₂₀, P₁₂₁ forming a by-pass of said

VSC

110, 111, 120, 121 when said by-

pass breaker

112, 113, 122, 123 is closed.

The HVDC transmission system 1 further includes a controller 15, being adapted to control current following through the closed by-pass breaker to reach zero and subsequently control the closed by-pass breaker to open. For example, the controller 15 includes a main controller 150 and

local controllers

1510, 1511, 1520, 1521, each of which is operatively associated with a

corresponding VSCs

110, 111, 120, 121. Under their control, the VSC associated therewith starts operation when it has been blocked and stops operation of that VSC when it has been unblocked.

It is assumed that one VSC of the first converter station 11, namely the VSC 110 has been blocked for maintenance or what is so ever, and the current path P₁₁₀ has been conducting with the closed by-pass breaker 112, and the VSC 110 is to be taken into operation, the main controller 15 may send de-blocking instruction to the local controller 1510 associated with the VSC 100. Further assuming that the

other VSCs

111, 120, 121 have been in operation with their associated by-

pass breakers

113, 122, 123 being opened blocking the current path P₁₁₁, P₁₂₀, P₁₂₁.

Figure 2 shows a block diagram of the local controller according to the first embodiment of present invention. As shown in figure 2, the local controller includes a first input terminal 20 for receiving voltage reference, a second input terminal 21 for receiving harmonic voltage reference, a adder 22 for adding the inputs from both of the

input terminals

20, 21, and an output terminal 23 for outputting the adding result to an PWM generator which in turn generates PWM signal to the associated VSC approximating the addition of the voltage reference and the harmonic voltage reference. Upon receiving the de-blocking instruction from the main controller 15, the local controller 1510 for the VSC 110 is then taking over the control of the VSC 110 by deblocking it by starting to control it to generate harmonic voltage at its DC side so as to inject zero-crossing harmonic current flowing through the closed by-pass breaker 112 via the conducting current path P₁₁₀. In order to generate harmonic voltage at the associated VSC’s DC side, the voltage reference may be set as zero and the harmonic voltage reference is set at n-th harmonic. By switching voltage to the associated VSC with the appropriate duty cycle, the output of the PWM generator will approximate a voltage at the harmonic voltage reference. The harmonic voltage generated at the DC side of the VSC 110 applies to the associated by-pass breaker 112 with substantially zero DC component, causing harmonic current flowing through the by-pass breaker 112 with zero-crossings. Namely, the closed by-pass breaker is controlled to open at the zero-crossing of the harmonic current. After confirming that the current in the by-pass breaker 112 is having zero-crossings making it possible to open the by-pass breaker, an opening order to this by-pass breaker 112 is given by the local controller 1510. To the right part of figure 2, it shows a wave form of the current flowing through the by-pass breaker according to the first embodiment. As shown in figure 2, the vertical axis indicates the by-pass breaker current I_by-pass, and the horizontal axis indicates time. From time period from t₀ to t₁ where the VSC 110 is blocked with its associated by-pass breaker 112 being closed, and the by-pass breaker is conducting the DC current I_DC. From time period from t₁ to t₂ where the VSC 110 applies the harmonic voltage at its DC side injecting the harmonic current into the by-pass breaker 112, the by-pass breaker current deviates from the DC current I_DC towards zero with the harmonics. From time period from t₂ to t₃, the by-pass breaker current appears oscillation around zero； at the time point t₃ it reaches one of the zero-crossing, the by-pass breaker 112 is opened and the VSC 110 is taken in. After the time point t₃, the by-pass breaker 112 maintains open conducting substantially zero current.

After the by-pass breaker has confirmed the open indication, the voltage reference of the local controller 1510 is changed to that for normal operation, and its harmonic reference is set to zero. Consequently, the PWM generator will generate PWM with reference to the voltage reference, and the delay angle of the VSC is adjusted accordingly. The VSC will continue in normal operation after the successful opening of its associated by-pass breaker. The HVDC converter system may include a first current measurement device 16, for measuring a first value of current flowing through the HVDC network, and a second current measurement device 1610, 1611, 1620, 1621, for measuring a second value of current flowing through the blocked

VSC

110, 111, 120, 121.

Following the above assumption where one VSC of the first converter station 11, namely the VSC 110 has been blocked, and the current path P₁₁₀ has been conducting with the closed by-pass breaker 112, the associated local controller 1510 may calculate a difference between the first value and the second value indicating a third value of the harmonic current flowing through the closed by-pass breaker 112 so as to determine the zero-crossing of the harmonic current.

As above, the first embodiment of present invention is described with example where the VSC, which has been blocked, operates as the VSC generating the harmonic voltage under the control of the controller.

As an alternative, the de-blocked VSC may operate as the VSC generating the harmonic voltage under the control of the controller, with a combination of the harmonic voltage and its operating DC voltage at its DC side. The frequency of the “zero crossing generator” oscillation can be chosen according to actual needs, for instance an inter-harmonic frequency is fully possible. Example will be described with the assumption where one VSC of the first converter station 11, namely the VSC 110 has been blocked, and another VSC, namely the VSC 111 has been in operation (de-blocked) , and the DC current has been flowing through the current path P₁₁₀ through the closed by-pass breaker 112 and the operating VSC 111. When the VSC 111 is controlled by its local controller 1511 to generate harmonic voltage at its DC side, since the DC voltage on the HVDC network is maintained substantially constant, there will be a harmonic voltage induced on the closed by-pass breaker 112 with polarity reverse to that of the harmonic voltage at the DC side of VSC 112. Consequently, a harmonic current will be injected in the he current path P₁₁₀ through the closed by-pass breaker 112, and the closed by-pass breaker 112 can be opened at the zero-crossing of its current. The skilled person should understand that both of the blocked VSC and the de-blocked VSC may operate as the VSC generating the harmonic voltage under the control of the controller.

In some scenario where the VSC to be taken in is not allowed to be de-blocked to generate the harmonic voltage or cannot operate to generate the harmonic voltage in sufficient magnitude for injection minimum harmonic current to its associated by-pass breaker, by having other VSC, besides the to-be taken in, generate the harmonic voltage in its replacement or in combination of it, the by-pass breaker can be opened in a reliable way, as well.

Figure 3 shows an HVDC transmission system according to a second embodiment of the present invention. In order to avoid redundancy and keep concise, the second embodiment is described based on that for the first embodiment.

The skilled person shall understand that VSC (voltage source converter) is made up of force turned-off semiconductor device, such as IGBT and IGCT, for which both turn-on and turn-off can be controlled, giving a second degree of freedom. In such converters, the polarity of DC voltage is usually fixed and the DC voltage, being smoothed by a large capacitance, can be considered constant. For allowance of reverse current flow, the force-turned-off semiconductor device is normally provided in parallel with an additional diode (freewheeling diode) in its assembly to conduct current in the opposite direction. VSC has several different configurations, for example including two-level VSC, three-level VSC, and modular multi-level VSC (MMC) .

In the receiving end of the HVDC transmission system, the

VSCs

120, 121 of the second converter station 12 operate in a mode of inverter； while in the sending end, the VSCs 110, 111 of the first converter station 11 operate in a mode of rectifier.

It is assumed that

VSCs

110, 111, 120, 121 have been in operation with their associated by-

pass breakers

112, 113, 122, 123 being opened blocking the current path P₁₁₀, P₁₁₁, P₁₂₀, P₁₂₁. Further, one VSC of the second converter station 12, namely the VSC 120 will be blocked for maintenance and its opened by-pass breaker 122 will be closed allowing conducting the P₁₂₀ so that the HVDC transmission system will be able to operate with the rest of the

VSCs

110, 111, 121. Here, the VSC 120 adopts the configuration of two-level converter as an example for describing the present invention. The two-level VSC 120 has a six pulse bridge with each leg having series-connected force turned-off

semiconductor devices

1200, 1201, 1202, 1203, 1204, 1205 each with inverse-parallel freewheeling diode D0, D1, D2, D3, D4, D5, and DC smoothing capacitor C is provided at its DC side.

Under such scenario, there will be a short circuit between positive and negative terminals of the DC side of VSC 120, which involves the freewheeling diodes D0, D1, D2, D3, D4, D5 of the VSC 120, and the closed by-pass breaker 122. This results in an excessive electric current limited only by the relatively small resistance of the network and potentially causes circuit damage, overheating, fire or explosion. Arrow P_sc indicates the short circuit path.

In order to prevent the short circuit occurring in the period of taking-outing one of the VSC on the receiving end of the HVDC transmission system, for example the VSC 120, the converter system of the second converter station 12 each further includes a

power diode

30, 31. As shown in figure 3, the power diode 30 is inserted in the connection between DC side of the VSC 120 and its associated by-pass breaker 122, and the forward direction of the power diode 30 is against the forward direction of the freewheeling diode of the VSC 120； the power diode 31 is inserted in the connection between DC side of the VSC 123 and its associated by-pass breaker 123, and the power diode 31 is against the forward direction of the freewheeling diode of the VSC 121. In summary, the power diode is inserted between the by-pass breaker and its corresponding VSC blocking short current loop involving the closed by-pass breaker and the freewheeling diodes of the force turned-off semiconductor device of its corresponding VSC, which may behave the power diode placed on the DC line for DC fault handling.

Therefore, the short circuit path P_sc will be blocked by the power diode 30 since their directions are opposite to each other during the taking-out of the VSC 120, while the

other VSCs

110, 111, 121 may still operate thus the HVDC transmission system will not be suspended. Besides, due to the fact that most HVDC transmissions are just transmitting in one direction, DC fault handling in Hybrid HVDC systems by means of a large diode placed on the DC transmission line may be used. As the

power diode

30, 31 may handle the DC fault as well, the large power diode for DC fault handling used in the transmission line of conventional HVDC transmission system may be removed.

Assuming that the VSC 120 has been blocked and will be taken in again, because the power diode 30 will completely block the harmonic current as injected according to the first embodiment, the zero-crossing of the current is not achievable making opening of the associated by-pass breaker 122 not feasible. In contrast according to the second embodiment, upon receiving the de-blocking instruction from the main controller 15, the

local controllers

1510, 1511 for the VSCs 110, 111 of the first converter station 11 at the sending end will block their VSCs 110, 111 for a predetermined time period, for example some hundreds of milliseconds, during which the DC current following in the HVDC network 10 drops to substantially zero； then, the local controller 1520 for the VSC 120 is taking over the control of the VSC 120 by open the associated by-pass breaker 122, rather than generating harmonic voltage at its DC side as disclosed in the first embodiment. Namely, the current following through the closed by-pass breaker is controlled to reach zero and subsequently the closed by-pass breaker is controlled to open.

Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no way limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

An HVDC converter system, including:

a plurality of VSCs being coupled at their DC sides in series, wherein two terminals of the series connection are configured to be coupled with HVDC network；

a plurality of by-pass breakers, each being in parallel coupled with the DC side of the corresponding one of the plurality of VSCs and being closed where its corresponding VSC is blocked； and

a controller, being adapted to control current following through the closed by-pass breaker to reach zero and subsequently control the closed by-pass breaker to open.
The HVDC converter system according to claim 1, wherein:

the controller is further adapted for controlling at least one of the plurality of VSCs to generate harmonic voltage at its DC side so as to inject zero-crossing harmonic current flowing through the closed by-pass breaker, and controlling the closed by-pass breaker to open at the zero-crossing of the harmonic current.
The HVDC converter system according to claim 1 or 2, further including:

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

a second current measurement device, for measuring a second value of current flowing through the blocked VSC；

wherein:

the controller is further adapted for calculating a difference between the first value and the second value indicating a third value of the harmonic current flowing through the closed by-pass breaker so as to determine the zero-crossing of the harmonic current.
The HVDC converter system according to any of the preceding claims 1 to 3, wherein:

the blocked VSC operates as the VSC generating the harmonic voltage under the control of the controller.
The HVDC converter system according to any of the claims 1 to 3, wherein:

the de-blocked VSC operates as the VSC generating the harmonic voltage under the control of the controller, with a combination of the harmonic voltage and its operating DC voltage at its DC side.
The HVDC converter system according to any of the preceding claims 1 to 3, wherein:

both of the blocked VSC and the de-blocked VSC operate as the VSC generating the harmonic voltage under the control of the controller.
The HVDC converter system according to any of the preceding claims, wherein:

the harmonic voltage is of n-th harmonic, n is an odd number.
The HVDC converter system according to claim 1, wherein:

the plurality of VSC are based on force turned-off semiconductor device, being configured to operate as receiving end of the HVDC network； and

the controller is further adapted for controlling to block sending end of the HVDC network for a predetermined period of time.
The HVDC converter system according to claim 8, further including:

a power diode, being inserted between the by-pass breaker and its corresponding VSC blocking short current loop involving the closed by-pass breaker and the freewheeling diodes of the force turned-off semiconductor device of its corresponding VSC.
A HVDC system using the HVDC converter system according to any of the preceding claims.
A method for controlling HVDC converter system, wherein:

the HVDC converter system includes:

a plurality of VSCs being coupled at their DC sides in series, wherein two terminals of the series connection are configured to be coupled with HVDC network； and

a plurality of by-pass breakers, each being in parallel coupled with the DC side of the corresponding one of the plurality of VSCs and being closed where its corresponding VSC is blocked；

the method including:

(a) controlling current following through the closed by-pass breaker to reach zero； and

(b) controlling the closed by-pass breaker to open subsequent to the step (a) .
The method according to claim 11, wherein:

the step (a) further includes:

controlling at least one of the plurality of VSCs to generate harmonic voltage at its DC side so as to inject zero-crossing harmonic current flowing through the closed by-pass breaker； and

the step (b) further includes:

controlling the closed by-pass breaker to open at the zero-crossing of the harmonic current.
The method according to claim 11, wherein:

the plurality of VSC are based on force turned-off semiconductor device, being configured to operate as receiving end of the HVDC network； and

the step (a) further includes:

controlling to block sending end of the HVDC network for a predetermined period of time.
The method according to claim 13, further including:

blocking short current loop involving the closed by-pass breaker and the freewheeling diodes of the force turned-off semiconductor devices of its corresponding VSC.