WO2017210892A1

WO2017210892A1 - Line-commutated converter control system and method

Info

Publication number: WO2017210892A1
Application number: PCT/CN2016/085264
Authority: WO
Inventors: Mats Andersson; Rong CAI; Hailian XIE; Lidong ZHANG
Original assignee: Abb Schweiz Ag
Priority date: 2016-06-08
Filing date: 2016-06-08
Publication date: 2017-12-14
Also published as: CN108701998B; CN108701998A

Abstract

It provides a control system for LCC of an HVDC system and a control method therefor. The control system includes: a first measurement unit (20) being configured to provide a current magnitude measurement indicative of a magnitude of DC current of the HVDC system; and a controller (21) being configured to increase a first extinction angle at which the controlled LCC operates to a second extinction angle in response to that the current magnitude measurement exceeds a current reference level in order to avoid commutation failure of the controlled LCC. By using the solutions according to the HVDC system with increase of extinction angle of the LCC, it is possible to decrease the probability of commutation failures caused by a rapid rise of the DC current of the HVDC system. The overall AC and DC power system reliability and stability is improved.

Description

LINE-COMMUTATED CONVERTER CONTROL SYSTEM AND METHOD

Technical Field

The invention relates to control of line-commutated converter (LCC) for high voltage direct current (HVDC) system, and more particularly to prevention of commutation failure of LCC of HVDC system.

Background Art

HVDC systems are commonly employed for interconnecting high voltage AC networks or a distant generating station to an AC network. Such systems typically consist of two converter stations interconnected by DC transmission line or cable. At the generating or transmitting end, the converter comprises a rectifier for rectifying the alternating power to direct current and voltage while at the receiving end the converter comprises an inverter which transfers power from the DC transmission line to the AC network.

A typical HVDC system may include a monopolar transmission line or bipolar transmission line. Figures 1A and 1B show the block diagrams of a bipolar HVDC system and a monopolar HVDC system. As shown in figure 1A, in the bipolar HVDC system, in the event that one of the poles in the bipolar system is faulted, the current path will switch to ground return, thus allowing the system to continue operating at reduced capacity and reducing the possibility that a pole fault will cause a bipolar outage. Usually these ground return paths are only used for very short durations until the faulted pole can be returned to service. The ground current in such schemes can flow in either direction, and the electrodes have to be designed to be reversible, operating either as an anode or cathode. In a three phase AC system, each phase of the transmitting and receiving AC networks is connected to the positive and negative DC transmission lines by two pair of valves oriented in the forward direction. As shown in figure 1B, in the monopolar HVDC system, the DC transmission line permanently carries the DC current. Each phase of the transmitting and receiving AC networks is connected to the DC transmission lines and the ground by valves oriented in the forward direction. The valves are actuated by a firing control system which provides gate signals to the valves in a predetermined time sequence to effect current transfer or commutation from phase to phase.

It is known that either types of the HVDC system may include converters comprising bridges based on thyristor or mercury arc valves. Such valve bridges permit the conversion of three phase alternating voltage and current to direct voltage and current or the inversion of direct voltage and current to three phase alternating voltage and current. Valves, such as thyristors, conduct current only in the forward direction from anode to cathode and only when the forward voltage across the valve is positive and the valve receives a control pulse. Once the valve has started to conduct, the magnitude of the current is determined solely by the main circuits outside the valve and is not influenced by removal of the gate pulse. The flow of current through the valve continues until it decreases as a result of external influences and attempts to become negative. Reverse current flow is prevented because the valve would be reverse biased so that the current through it is extinguished. In the forward direction, the valve will block current flow until a control pulse is applied to the gate. As a result of these properties, the operating cycle of a valve is divided into a forward blocking interval, a conducting interval and a reverse blocking interval.

When the valves are operated in the inverter mode, the direct voltage is negative when referred to current direction. This means that the voltage across the valves is positive most of the time before a firing pulse is applied. To establish a forward blocking voltage, the charge established during the conducting period must be removed. Therefore, the valve requires a time interval with a negative valve voltage between the end of the conducting period and the application of positive voltage. The electrical angle corresponding to this time period is called the margin of commutation or the extinction angle.

In typical inverter operation, with one valve conducting, the firing of the next succeeding valve is ordered in sufficient time before the next zero crossing, at which time the phase-to-phase voltage will become positive. Thus, the commutation from the out-going valve to the incoming valve must be finalized in time to insure a sufficient commutation margin. If for some reason commutation is not finished when the voltage across the out-going valve become positive or the commutation margin is so small that the valve does not have time to regain sufficient forward blocking capability, there is a transient disturbance in the inverter operation known as commutation failure.

As noted above, to establish the forward blocking capability of a valve, the charges established during the conduction interval are removed by providing a negative valve voltage for a time interval corresponding to the commutation time. Since rectifiers are normally operated at firing angles of less than ninety electrical degrees, this represents no problem in rectifier operation. However, such commutation failures are a concern with inverter operation because of the desirability of maintaining the extinction angle as small as possible to maximize power transfer.

Conventional inverter firing angle control systems normally attempt to prevent commutation failure by increasing of commutation margin in response to decreased AC network voltage. Such method for mitigation of commutation failure is disclosed in ” A Novel method to mitigate commutation failures in HVDC systems” , Lidong Zhang, Lars

PowerCon 2002, Kunming, China, October 13 -17, 2002. According to the paper, classical HVDC system is susceptible to commutation failures at the inverter station. This is because successful thyristor turn-off depends on the external AC network voltage. The existing solution applied to this problem, uses the external AC network voltage as its key input, and will if necessary make a temporary increase in the inverter extinction angle in order to avoid commutation failures.

It is known that in the monopolar HVDC system, if the rectifier AC voltage rises rapidly, the DC current will also rise rapidly until the rectifier current controller brings down the measured DC current to its ordered value. Before the rectifier has decreased the DC current, the inverter overlap angle (volt-time area) will increase. Due to the increase in the volt-time area, the inverter might suffer from a commutation failure. As for bipolar HVDC system, there will be inductive mutual coupling between the two DC transmission lines. As soon as the DC current changes rapidly in one pole, there will also be a change in the other pole. This phenomenon can cause a commutation failure in an already operating pole, when the other pole restart after clearing a DC transmission line fault. Therefore, there is still a need for improvement in regulation of the extinction angle in order to better mitigate the commutation failure when there is a rise of the DC current in the DC transmission line.

Brief Summary of the Invention

According to one aspect of present invention, it provides a control system for LCC of an HVDC system, including； a first measurement unit being configured to provide a current magnitude measurement indicative of a magnitude of DC current of the HVDC system； and a controller being configured to increase a first extinction angle at which the controlled LCC operates to a second extinction angle in response to that the current magnitude measurement exceeds a current reference level in order to avoid commutation failure of the controlled LCC.

According to another aspect of present invention, it provides a control method for LCC of an HVDC system, including providing a current magnitude measurement indicative of a magnitude of DC current of the HVDC system； and increasing a first extinction angle at which the controlled LCC operates to a second extinction angle in response to that the current magnitude measurement exceeds a current reference level in order to avoid commutation failure of the controlled LCC.

By using the solutions according to present invention with increase of extinction angle of the LCC, it is possible to decrease the probability of commutation failures caused by a rapid rise of the DC current of the HVDC system. The overall AC and DC power system reliability and stability is improved.

Preferably, the controller is further configured to predict an overlap angle of the controlled LCC being excess over minimum required for successful commutation in consideration of the current magnitude measurement； and decrease a firing angle of the controlled LCC so as to achieve the increase of the first extinction angle to the second extinction angle and the predicted overlap angle of the controlled LCC. As regards the method, it is preferable to include steps of predicting an overlap angle of the controlled LCC being in excess of minimum required for successful commutation in consideration of the current magnitude measurement； wherein: decrease a firing angle of the controlled LCC so as to achieve the increase of the first extinction angle to the second extinction angle and the predicted overlap angle of the controlled LCC. A firing angle of the LCC, thereby, may be decreased so as to leave room for an increase of the first extinction angle and an enlargement of volt-time area of the controlled LCC. By the regulation of the firing angle of the valves, a first extinction angle at which the controlled LCC operates may be increased to a second extinction angle in response to that the current magnitude measurement of the DC current exceeds a current reference level in order to avoid commutation failure of the controlled LCC. The volt-time area A may be regulated in excess of the minimum required for successful commutation, as well.

In particular where the HVDC system is a bipolar HVDC system including a first pole HVDC transmission line and a second pole HVDC transmission line, wherein an inductive mutual coupling exists therebetween； a DC side of the controlled LCC is coupled to the first pole HVDC transmission line； the first measurement unit is configured to provide the current magnitude measurement indicative of the magnitude of the DC current in the second pole HVDC transmission line of the HVDC system； and the current reference level is set at substantial zero during a period when the second pole is restarted until it resumes the normal operation. As soon as the DC current changes rapidly in the second pole HVDC transmission line, there will also be a change in the DC current in the first pole HVDC transmission line due to the inductive mutual coupling. This phenomenon can cause a commutation failure in the already operating first pole, when the second pole restarts after clearing a DC line fault. The first measurement unit of the control system continuously measures the DC current in the second pole HVDC transmission line. When the rectifier of the second pole resumes DC power transmission again, DC current will flow in the second pole HVDC transmission line. As soon as the first measurement unit of control system measures a high enough DC current, it will make a temporary increase in its extinction angle as described above in order to avoid commutation failures.

Preferably, the controller is further configured to decrease from the second extinction angle to the first extinction angle substantially at minimum required for successful commutation during the period. By keeping the extinction angle at the minimum required for successful commutation, this allows to meet the desirability of regulating the extinction angle as small as possible to maximize power transfer.

In particular where the HVDC system is a monopolar HVDC system； a DC side of the controlled LCC is coupled to the HVDC transmission line； the first measurement unit is configured to provide the current magnitude measurement indicative of the magnitude of the DC current in the HVDC transmission line of the HVDC system； and the current reference level is set at a steady state value formed by low pass filtering the magnitude of the DC current in normal operation. As soon as the first measurement unit of control system measures a high enough DC current, it will make a temporary increase in its extinction angle as described above in order to avoid commutation failures. In addition, preferably, the control system further includes a second measurement unit being configured to provide a voltage magnitude measurement indicative of a magnitude of DC voltage of the HVDC system； wherein: the controller is further configured to increase the first extinction to the second extinction angle in response to that the voltage magnitude measurement exceeds a voltage reference level in order to avoid commutation failure of the controlled LCC； and the voltage reference level is set at a steady state value formed by low pass filtering the magnitude of the DC voltage in normal operation.

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:

Figures 1A and 1B show the block diagrams of a bipolar HVDC system and a monopolar HVDC system；

Figures 2A and 2B respectively illustrate a block diagram of a control system for LCC of HVDC system and operation of control system according to an embodiment of present invention；

Figure 2C illustrates commutation process of the LCC according to an embodiment of present invention.

Figure 3A illustrates block diagram of a bipolar HVDC system using the control system according to an embodiment of present invention；

Figure 3B illustrates the waveforms of the extinction angle addition and the DC current in the second pole HVDC transmission line according to the embodiment of the bipolar HVDC system；

Figure 4A illustrates block diagram of a monopolar HVDC system using the control system according to an embodiment of present invention； and

Figure 4B illustrates the waveforms of the extinction angle addition and the DC current in the HVDC transmission line according to the embodiment of the monopolar HVDC system.

Preferred Embodiments of the Invention

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and programming procedures, devices, and circuits are omitted so not to obscure the description of the present invention with unnecessary detail.

Figures 2A and 2B respectively illustrate a block diagram of a control system for LCC of HVDC system and operation of control system according to an embodiment of present invention. Figure 2C illustrates commutation process of the LCC according to an embodiment of present invention.

As shown in figure 2A, the control system 2 includes a first measurement unit 20 and a controller 21. In this embodiment, the first measurement unit 20 may be shunt resistor, Hall Effect current sensor transducer, and transformer, having its current detection part coupled to the DC transmission line of the HVDC system. The first measurement unit 20 may provide a current magnitude measurement I_{DC_M} indicative of a magnitude of DC current I_DC of the HVDC system. The detection is based on instantaneous current values, which ensures fast reaction of the control system when a relatively rapid rise of the DC current occurs. Since the LCC transformer has inductance, the transformer current cannot change instantly. The finite rate of change of current means that the transfer of current from one valve to another requires a finite commutation time.

As shown in figure 2C, the commutating current sources from the DC current of the HVDC system. For example, before commutation when just

valves

1 and 2 are conducting, the DC voltage is formed from two of the three phase voltages. During the overlap period where current I (indicated by the arrows) is commutated from valve 1 (the out-going valve) to valve 3 (the incoming valve) , the DC voltage is formed from all three phase voltages. The higher the commutating current, the larger the volt-time area A will be. Typical full load values of overlap angle μ are in the range 20 to 30 electrical degrees under normal steady-state operation. A phenomenon in thyristor valves is that the internal stored charges produced during a forward conduction interval must be removed before the valve can establish a forward voltage blocking capability. This time is known as the de-ionisation time of the valve, and the time from the instant when the valve current goes to zero to the time that the line-to-line voltage is zero is defined as the extinction angle γ. If a thyristor becomes positively biased before complete de-ionisation occurs, this thyristor will regain current.

As shown in figure 2B, state I represents the normal steady-state operation without control by the control system 2 for prevention of commutation failure, while state II represents the transient-state operation where the commutation failure prevention is applied. The firing angle, overlap angle and extinction angle for states I and II are respectively indicated by α₁, μ₁, γ₁, and α₂, μ₂, γ₂. The extinction angle γ₁, γ₂ is illustrated in figure 2B as the remaining voltage-time area of the commutation voltage after commutation, from the end at the volt-time area to the instant when the voltage across the valve changes sign, that is, from reverse to blocking voltage. This illustrates that successful commutation requires a minimum voltage-time area rather than a fixed time interval.

It is known that the electrical angle of the volt-time area A, namely the overlap angle μ, is required for the commutation. The overlap angle μ is related to the commutating current flowing through the valves in commutation. Since the overlap angle μ increases with the increased DC current, a relatively larger overlap angle μ will be needed to complete the commutation. The extinction angle γ is indicated in figure 2B as the horizontally shaded area and is calculated by the controller 21 from the following equation:

180°＝α+μ+γ (1)

where: α is the firing angle of the incoming valve, μ is the overlap angle involving the incoming valve and the out-going valve, and γ is the extinction angle of the out-going valve. Too small an extinction angle γ due to a rapid rise of the DC current is one of the basic reasons of commutation failures. In order to keep a big enough γ of the out-going valve, the controller 21 should give an advanced firing instant on detection of the DC current disturbance, while leaving room for an increase of the volt-time area A for completion of the de-ionisation of the out-going valve. The volt-time area A is related to the commutating current flowing through the valves in commutation, the volt-time area A being the excess over the minimum required for successful commutation may be predicted in consideration of the current magnitude measurement indicative of a magnitude of DC current of the HVDC system as provided by the first measurement unit 20. As shown in figure 2B, in normal steady-state operation I, a margin area will exist. However, should the firing angle α be inadequate to maintain a sufficient voltage-time area A and the extinction angle γ due to a rapid rise of the DC current, a firing angle calculation module 211 of the controller 20 will decrease the firing angle α, for example from α₁ to α₂, or in other words, the firing angle αwill be reduced so that the voltage-time area A between firing and the zero crossing and the extinction angle γ will be increased, for example from μ₁ to μ₂ and from γ₁, to γ₂.

As described above, the volt-time area A may be in excess of the minimum required for successful commutation. The overlap angle μ may be predicted from the following equation by an overlap angle prediction module 210 of the controller 21:

wherein: I_d is the measured DC current, I_dN is nominal DC current, U_dioN is nominal no load DC voltage, and U_dio is measured no load DC voltage. Besides, the horizontally shaded area in figure 2B is typically set between 17 to 18 electrical degrees, which will result in commutation under normal conditions. In order to ensure that successful valve commutation occurs, the firing control signal must be delivered to the valve at a firing angle of 180 degrees minus 17 to 18 electrical degrees minus the overlap angle or less.

In this embodiment, following the numerical relation as defined in equation (1) , a firing angle calculation module 211 of the controller 21 is configured to decrease a firing angle αof the valves of the controlled LCC so as to achieve an increase of the first extinction angle γ and an enlargement of volt-time area A of the controlled LCC. By the regulation of the firing angle α of the valves, a first extinction angle γ1 at which the controlled LCC operates may be increased to a second extinction angle γ2 in response to that the current magnitude measurement of the DC current exceeds a current reference level in order to avoid commutation failure of the controlled LCC. The volt-time area A may be regulated in excess of the minimum required for successful commutation, as well.

By using the solution according to the embodiment of present invention, it is possible to decrease the probability of commutation failures caused by a rapid rise of the DC current of the HVDC system. The overall AC and DC power system reliability and stability is improved.

Figure 3A illustrates block diagram of a bipolar HVDC system using the control system according to an embodiment of present invention. Figure 3B illustrates the waveforms of the extinction angle addition and the DC current in the second pole HVDC transmission line according to the embodiment of the bipolar HVDC system.

As shown in figure 3A, the bipolar HVDC system 3 includes a first pole HVDC transmission line 30, a second pole HVDC transmission line 31 and the control system 2. The first pole HVDC transmission line 30 and the second pole HVDC transmission line 31 each is at a high potential with respect to ground, in opposite polarity. They are coupled between two stations A, B made up of LCCs R1, R2, I1, I2. LCCs R1, I1 and the first pole HVDC transmission line 30 constitute the first pole with LCC R1 operating as rectifier and LCC I1 operating as inverter, and LCCs R2, I2 and the second pole HVDC transmission line 31 constitute the second pole with LCC R2 operating as rectifier and LCC I2 operating as inverter. There exists an inductive mutual coupling L between the first pole HVDC transmission line 30 and the second pole HVDC transmission line 31.

As shown in figure 3A, in this embodiment, DC side of the LCC I1 is coupled to the first pole HVDC transmission line 30, which is put under the control of the control system 2 for prevention of commutation failure. The control system 2 may be coupled to the valves of the LCC I1 so as to regulate their firing angles α. The first measurement unit 20 of the control system 2 is arranged besides the second pole HVDC transmission line 31 for provide the current magnitude measurement indicative of the magnitude of the DC current in the second pole HVDC transmission line 31 of the HVDC system. As soon as the DC current changes rapidly in the second pole HVDC transmission line 31, there will also be a change in the DC current in the first pole HVDC transmission line 30 due to the inductive mutual coupling L. This phenomenon can cause a commutation failure in the already operating first pole, when the second pole restarts after clearing a DC line fault. The first measurement unit 20 of the control system 2 continuously measures the DC current in the second pole HVDC transmission line 31. When the rectifier R2 of the second pole resumes DC power transmission again, DC current will flow in the second pole HVDC transmission line 31. As soon as the first measurement unit 20 of control system 2 measures a high enough DC current, it will make a temporary increase in its extinction angle γ as described above in order to avoid commutation failures. In the situation of restarting the second pole, the rapid rise of the DC current occurs in the first pole HVDC transmission line 30 on the initiation of the rise of the DC current in the second pole HVDC transmission line 31 from substantially zero. In order to identify the situation, the current reference level may be set at substantial zero during a period when the second pole is restarted until it resumes the normal operation. As shown in figure 3B, the DC current in the second pole HVDC transmission line 31 becomes more flat with it approaches value of the normal steady-state operation. As a result, the DC current generated in the first pole HVDC transmission line 30 from the inductive mutual coupling L decreases from the initiation of the restart of the second pole to the normal operation. In this embodiment, the controller 2 is further configured to gradually decrease from the second extinction angle γ2 to the first extinction angle γ1 during the period when the second pole is restarted until it resumes the normal operation. The regulation of the second extinction angle γ2 may be expressed as a step response followed an exponential decay according to the equation below:

N (t) ＝N₀*e ^(-λt) (3)

wherein: N (t) is the added extinction angle γ2 minus γ1, N₀is the added extinction angle at the initiation of the restart of the second pole, , λ is the decay time constant.

By keeping the extinction angle at the minimum required for successful commutation, this allows to meet the desirability of regulating the extinction angle as small as possible to maximize power transfer.

Figure 4A illustrates block diagram of a monopolar HVDC system using the control system according to an embodiment of present invention. Figure 4B illustrates the waveforms of the extinction angle addition and the DC current in the HVDC transmission line according to the embodiment of the monopolar HVDC system.

As shown in figure 4A, the monopolar HVDC system 4 includes a DC current transmission line 40 and the control system 2. The DC current transmission line 40 is coupled between two stations A, B made up of LCCs R, I with LCC R operating as rectifier and LCC I operating as inverter.

As shown in figure 4A, in this embodiment, DC side of the LCC I is coupled to the HVDC transmission line 40 and the ground, which is put under the control of the control system 2 for prevention of commutation failure. For example, the control system 2 may be coupled to the valves of the LCC I so as to regulate their firing angles α. The first measurement unit 20 of the control system 2 is arranged besides the HVDC transmission line 40 for provide the current magnitude measurement indicative of the magnitude of the DC current in the HVDC transmission line 40 of the HVDC system. An HVDC system fault, such as a sharp rise of AC voltage at the LCC R, may result in that the DC current changes rapidly in the HVDC transmission line 40. This phenomenon can cause a commutation failure in the LCC I operating as an inverter. As soon as the first measurement unit 20 of control system 2 measures a high enough DC current, it will make a temporary increase in its extinction angle γ as described above in order to avoid commutation failures. In order to identify the situation, the current reference level may be set at a steady state value formed by low pass filtering the magnitude of the DC current in normal operation.

In this embodiment, the control system 2 may further include a second measurement unit 22 being configured to provide a voltage magnitude measurement indicative of a magnitude of DC voltage of the HVDC system. For example, the second measurement unit 22 may be a voltmeter, a potentiometer, or an oscilloscope, and it may be arranged in a station accommodating the LCC I with its detecting ends coupled across the DC transmission line and the ground. The controller 21 is further configured to increase the first extinction γ1 to the second extinction angle γ2 in response to that the voltage magnitude measurement exceeds a voltage reference level in order to avoid commutation failure of the controlled LCC. The voltage reference level is set at a steady state value formed by low pass filtering the magnitude of the DC voltage in normal operation. The key advantage with also including the DC voltage measurement criterion, is to avoid spurious identification by only the DC current criterion.

If both the DC current and DC voltage increases above their corresponding reference levels at the same time, the commutation failure prevention may act as shown in figure 4B. In this embodiment, the controller 2 is further configured to gradually decrease from the second extinction angle γ2 to the first extinction angle γ1 during the period when the DC transmission resumes normal operation. The regulation of the second extinction angle γ2 may be expressed as a step response followed an exponential decay according to equation (3) .

In each of the embodiments of present invention, the increment value of the second extinction angle γ2 with respect to the first extinction angle γ1 ranges from 10 to 15 electrical degrees. This gives room for the increase of the overlap angle μ and for the valves to recover their ability to withstand positive voltage after conducting current.

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

A control system for LCC of an HVDC system, including；

a first measurement unit being configured to provide a current magnitude measurement indicative of a magnitude of DC current of the HVDC system； and

a controller being configured to increase a first extinction angle at which the controlled LCC operates to a second extinction angle in response to that the current magnitude measurement exceeds a current reference level in order to avoid commutation failure of the controlled LCC.
The control system according to claim 1, wherein:

the controller is further configured to

predict an overlap angle of the controlled LCC being excess over minimum required for successful commutation in consideration of the current magnitude measurement； and

decrease a firing angle of the controlled LCC so as to achieve the increase of the first extinction angle to the second extinction angle and the predicted overlap angle of the controlled LCC.
The control system according to any of the preceding claims, wherein:

the HVDC system is a bipolar HVDC system including a first pole HVDC transmission line and a second pole HVDC transmission line, wherein an inductive mutual coupling exists therebetween；

a DC side of the controlled LCC is coupled to the first pole HVDC transmission line；

the first measurement unit is configured to provide the current magnitude measurement indicative of the magnitude of the DC current in the second pole HVDC transmission line of the HVDC system； and

the current reference level is set at substantial zero during a period when the second pole is restarted until it resumes the normal operation.
The control system according to claim 3, wherein:

the controller is further configured to decrease from the second extinction angle to the first extinction angle substantially at minimum required for successful commutation during the period.
The control system according to any of the claims 1 to 2, wherein:

the HVDC system is a monopolar HVDC system；

a DC side of the controlled LCC is coupled to the HVDC transmission line；

the first measurement unit is configured to provide the current magnitude measurement indicative of the magnitude of the DC current in the HVDC transmission line of the HVDC system； and

the current reference level is set at a steady state value formed by low pass filtering the magnitude of the DC current in normal operation.
The control system according to claim 5, further including:

a second measurement unit being configured to provide a voltage magnitude measurement indicative of a magnitude of DC voltage of the HVDC system；

wherein:

the controller is further configured to increase the first extinction to the second extinction angle in response to that the voltage magnitude measurement exceeds a voltage reference level in order to avoid commutation failure of the controlled LCC； and

the voltage reference level is set at a steady state value formed by low pass filtering the magnitude of the DC voltage in normal operation.
The control system according to any of the preceding claims, wherein:

the increment value of the second extinction angle with respect to the first extinction angle ranges from 10 to 15 electrical degrees.
A control method for LCC of an HVDC system, including；

providing a current magnitude measurement indicative of a magnitude of DC current of the HVDC system； and

increasing a first extinction angle at which the controlled LCC operates to a second extinction angle in response to that the current magnitude measurement exceeds a current reference level in order to avoid commutation failure of the controlled LCC.
The method according to claim 8, further including:

predicting an overlap angle of the controlled LCC being in excess of minimum required for successful commutation in consideration of the current magnitude measurement；

wherein:

decrease a firing angle of the controlled LCC so as to achieve the increase of the first extinction angle to the second extinction angle and the predicted overlap angle of the controlled LCC.
The control method according to any of claims 8 to 9, wherein:

the HVDC system is a bipolar HVDC system including a first pole HVDC transmission line and a second pole HVDC transmission line, wherein an inductive mutual coupling exists therebetween；

a DC side of the LCC is coupled to the first pole HVDC transmission line；

the current magnitude measurement is indicative of the magnitude of the DC current in the second pole HVDC transmission line of the HVDC system； and

the current reference level is set at substantial zero during a period when the second pole is restarted until it resumes the normal operation.
The control method according to claim 10, wherein:

gradually decreasing from the second extinction angle to the first extinction angle during the period.
The control method according to any of the claims 8 to 9, wherein:

the HVDC system is a monopolar HVDC system；

a DC side of the LCC is coupled to the HVDC transmission line；

the current magnitude measurement is indicative of the magnitude of the DC current in the HVDC transmission line of the HVDC system； and

the current reference level is set at a steady state value formed by low pass filtering the magnitude of the DC current in normal operation.
The control method according to claim 12, further including:

providing a voltage magnitude measurement indicative of a magnitude of DC voltage of the HVDC system；

wherein:

the increase of the first extinction to the second extinction angle is done in response to that the voltage magnitude measurement exceeds a voltage reference level in order to avoid commutation failure of the controlled LCC； and

the voltage reference level is set at a steady state value formed by low pass filtering the magnitude of the DC voltage in normal operation.
The control method according to any of claims 8 to 13, wherein:

the increment value of the second extinction angle with respect to the first extinction angle ranges from 10 to 15 electrical degrees.