阅读说明：本技术 换流站的无通信控制 (Communication-less control of a converter station ) 是由 S·罗伊-乔杜里 英·江-赫夫纳 G-K·卡萨尔 M·穆罕默迪 马茨·安德森 于 2017-12-21 设计创作，主要内容包括：提供了用于操作HVDC传输系统中的换流站(120)的方法,该HVDC传输系统包括具有第一端(112)和第二端(114)的DC传输链路(110),在第一端处连接该换流站(120),并且在第二端处连接第二换流站(130)。该方法可以包括：在DC传输链路的第一端处感测DC传输链路的DC电流和DC电压；基于感测到的DC电流和感测到的DC电压来确定第二换流站(130)处是否已经发生相接地故障,以及基于(152)确定已经发生相接地故障而减少由该换流站(120)经由DC传输链路(110)输送至第二换流站(130)的功率。还提供了换流站和HVDC传输系统。(A method for operating a converter station (120) in an HVDC transmission system comprising a DC transmission link (110) having a first end (112) and a second end (114), the converter station (120) being connected at the first end and a second converter station (130) being connected at the second end is provided. The method can comprise the following steps: sensing a DC current and a DC voltage of the DC transmission link at a first end of the DC transmission link; determining whether a phase to ground fault has occurred at the second converter station (130) based on the sensed DC current and the sensed DC voltage, and reducing the power delivered by the converter station (120) to the second converter station (130) via the DC transmission link (110) based on (152) determining that a phase to ground fault has occurred. A converter station and a HVDC transmission system are also provided.)

1. A method of operating a first converter station in a high voltage direct current, HVDC, transmission system comprising a direct current, DC, transmission link having a first end and a second end, the first converter station being connected at the first end and a second converter station being connected at the second end, the method comprising:

sensing a DC current and a DC voltage of the DC transmission link at the first end of the DC transmission link;

determining whether a phase to ground fault has occurred at the second converter station based on the sensed DC current and the sensed DC voltage, an

Reducing power delivered by the first converter station to the second converter station via the DC transmission link based on the determination that the phase ground fault has occurred.

2. The method of claim 1, wherein determining that the phase-to-ground fault occurred comprises:

determining whether the sensed DC current is equal to or greater than a threshold current value;

determining whether at least partial recovery of the sensed DC voltage has occurred based on determining that the sensed DC current is equal to or greater than the threshold current value, an

Determining that the phase to ground fault has occurred based on determining that the at least partial recovery of the sensed DC voltage has occurred.

3. The method of claim 2, wherein determining the at least partial recovery of the sensed DC voltage comprises:

determining whether the sensed DC voltage is equal to or greater than a threshold voltage value, an

Determining that the at least partial recovery of the sensed DC voltage has occurred based on a determination that the sensed DC voltage is equal to or greater than the threshold voltage value.

4. The method of claim 3, wherein the threshold voltage value is a value of the sensed DC voltage at a first time, and wherein the sensed DC voltage is determined to be equal to or greater than a threshold voltage at least before a second time after the first time.

5. The method of claim 4, wherein the first time is at a time of determining that the sensed DC current is equal to or greater than the threshold current value, or wherein the first time is at a time of determining that the sensed DC current has assumed a maximum current value.

6. The method of claim 4 or 5, wherein the difference between the second time and the first time is equal to or less than about one cycle time of the AC frequency.

7. A method according to any of claims 3 to 6, wherein the threshold voltage value is defined as the value of the sensed DC voltage at a first time.

8. The method of any of claims 2 to 7, wherein determining the at least partial recovery of the sensed DC voltage comprises:

determining whether a time derivative of the sensed DC voltage is equal to or greater than a threshold voltage time derivative value, an

Determining that the at least partial recovery of the sensed DC voltage has occurred based on determining that the time derivative of the sensed DC voltage is equal to or greater than the threshold voltage time derivative value.

9. A method according to any of claims 2-8, wherein the threshold current value is proportional to the power stage number used for controlling the first converter station.

10. A converter station for a high voltage direct current, HVDC, transmission system comprising a direct current, DC, transmission link, wherein the converter station is connectable to a first end of the DC transmission link and comprises:

a current sensor for sensing a DC current at the first end of the DC transmission link;

a voltage sensor for sensing a DC voltage at the first end of the DC transmission link, an

A controller configured to:

determining whether a phase-to-ground fault has occurred at a second converter station connected to a second end of the DC transmission link based on the sensed DC current and the sensed DC voltage, and

controlling the converter station to reduce the power delivered by the converter station to the second converter station via the DC transmission link based on the determination that the phase ground fault has occurred.

11. A converter station according to claim 10, wherein the controller is further arranged to perform the method according to any of claims 2-9.

12. A converter station according to claim 10 or 11, wherein the converter station comprises at least one of the following converters: a line commutated converter LCC, a half-bridge modular multilevel converter HB MMC, and a full-bridge modular multilevel converter FB MMC.

13. An HVDC transmission system comprising: a first converter station according to any one of claims 10 to 12; a second converter station; and a DC transmission link connecting said first converter station and said second converter station.

14. The HVDC transmission system of claim 13, wherein said second converter station comprises at least one of a half-bridge modular multilevel converter (HB MMC) and a full-bridge modular multilevel converter (FB MMC).

15. The HVDC transmission system of claim 14, wherein the second converter station comprises at least a HB MMC and a FB MMC connected in series.

16. The HVDC transmission system of claim 15, wherein the FB MMC is connected to a DC line of the DC transmission link closer to over which power is transmitted from the first converter station to the second converter station than the HB MMC.

17. The HVDC transmission system of any of claims 13-16, wherein said first converter station comprises a line commutated converter, LCC.

Technical Field

The present disclosure relates to a method of operating a converter station in a High Voltage Direct Current (HVDC) transmission system. More generally, the present disclosure relates to a method of operating a second HVDC converter station connected to the HVDC converter station via a Direct Current (DC) transmission link during at least a phase to ground fault at such converter station.

Background

In HVDC transmission systems, the use of one or more Voltage Source Converters (VSCs) at the inverting (i.e. receiving) end of the DC transmission link may have advantages. Such advantages may include, for example, greater freedom in control, the ability to avoid commutation failures, and/or enhanced possibilities of connecting to, for example, passive Alternating Current (AC) loads.

However, during phase to ground faults occurring at the AC terminals of the VSC, the VSC may be subject to high over-voltage stresses. In order to avoid that such overvoltage stress leads to VSC damage, further protection devices may be installed. Additionally, it may be desirable to communicate the occurrence of such a fault to other units or converters in the same system in order to reduce (or stop) the power delivered to the failed VSC. Since this may lead to increased costs and overall complexity of the HVDC transmission system, there is a need for both an improved converter station and an improved method of operating a converter station.

Disclosure of Invention

The present disclosure seeks to at least partially meet the above needs. To achieve this, a method of operating a converter station, a converter station and an HVDC transmission system as defined in the independent claims are provided. Further embodiments of the disclosure are provided in the dependent claims.

According to a first aspect of the present disclosure, a method of operating a first converter station in an HVDC transmission system is provided. The HVDC transmission system may comprise a DC transmission link having a first end and a second end, the first converter station being connected at the first end and the second converter station being connected at the second end. The method can include sensing a DC current and a DC voltage of the DC transmission link at a first end of the DC transmission link. The method may further comprise determining whether a phase to ground fault has occurred at the second converter station based on the sensed DC current and the sensed DC voltage. The method can further comprise reducing the power delivered by the first converter station to the second converter station via the DC transmission link based on the determination that the phase to ground fault has occurred.

In conventional HVDC transmission systems, the determination of whether a phase to ground fault has occurred at the second converter station is performed at the second converter station, for example by locally measuring the voltage and current at the second converter station. If it is determined that a phase to ground fault has occurred, information indicating this fault needs to be transmitted to the first converter station (at the rectifying end of the DC transmission link). Then, first, after receiving this information, the first converter station may reduce the amount of power it delivers to the second converter station. Communicating this information at the appropriate time may require a fast and reliable communication channel between the switching stations (such as a radio network or internet cable). Establishing and maintaining such communication channels may increase the cost of the HVDC transmission system, and the reliability of the communication channels may affect the ability of the system to handle phase ground faults accordingly.

With the method of the present disclosure, sensing the DC voltage and the DC current and determining whether a phase to ground fault has occurred at the second converter station are performed at the first converter station. In other words, the detection of the occurrence of the phase to ground fault (at the second converter station) and the reduction of power (delivered to the second converter station) may be performed locally at the first converter station. This may eliminate the need for communication channels and may improve the HVDC transmission system in terms of reduced cost and increased reliability. In other words, the converter station may operate in a communication-less manner. A more reliable way of reducing or stopping the power delivered from the first converter station may also reduce the need for increasing the size of the second converter station. This may, for example, further reduce the cost of the system.

In some embodiments, determining the occurrence of a phase-to-ground fault may include determining whether the sensed DC current is equal to or greater than a threshold current value. Determining the occurrence of the phase ground fault may further include determining whether at least partial recovery of the sensed DC voltage has occurred based on determining that the sensed DC current is equal to or greater than a threshold current value. Determining the occurrence of the phase-to-ground fault may further include determining that a phase-to-ground fault has occurred based on determining that at least partial recovery of the sensed DC voltage has occurred.

"recovery of the sensed DC voltage" may be defined as the sensed DC voltage reaching its value before or near the occurrence of the phase-to-ground fault. Determining that the sensed DC current is equal to or greater than the threshold current value may provide an indication that a fault has occurred at the inverter side of the system (i.e., at the second converter station). It may be determined that the sensed DC voltage has at least partially recovered after determining that the sensed DC current is equal to or greater than the threshold current value. Determining that the sensed DC voltage has at least partially recovered may confirm that the indicated fault is a phase-to-ground fault and is distinguished from other faults, such as a DC bus fault and/or an external AC fault (e.g., on the inverter side of the system, i.e., at the second converter).

In some embodiments, determining at least partial recovery of the sensed DC voltage may include determining whether the sensed DC voltage is equal to or greater than a threshold voltage value. Determining at least partial recovery of the sensed DC voltage may further include determining that at least partial recovery of the sensed DC voltage has occurred based on determining that the sensed DC voltage is equal to or greater than a threshold voltage value.

In some embodiments, the threshold voltage value may be a value of the sensed DC voltage at a first time. In the method, it may also be determined that the sensed DC voltage is equal to or greater than the threshold voltage value at least before a second time after the first time. For example, the first time may be a time at or near a time at which the phase-to-ground fault occurred, and the threshold voltage value may be a value of the sensed DC voltage at a time before or near the time at which the phase-to-ground fault occurred (i.e., at the first time). The second time may define an end of a waiting period during which the sensed DC voltage value is expected to again reach (or exceed) the threshold value, which may confirm or at least indicate that at least partial recovery of the sensed DC voltage has occurred.

In some embodiments, the first time may be when it is determined that the sensed DC current is equal to or greater than a threshold current value. The time at which the sensed DC current is determined to be equal to or greater than the threshold current value may for example be considered as the time at which a potential phase to ground fault occurs at the second converter station. Alternatively, the first time may be at a time when it is determined that the sensed DC current has assumed the maximum current value. The time at which the sensed DC current assumes the maximum current value may for example be considered as the time indicating that a fault (possibly a phase-to-ground fault) has occurred at the inverter station, i.e. at the second converter station. In some embodiments, the first time may be at a time when it is determined that the sensed DC current has assumed the maximum current value, provided that the sensed DC current has reached or exceeded a threshold current value before the maximum current value has been assumed.

In some embodiments, the difference between the second time and the first time may be equal to or less than, for example, a time of about one cycle of the AC frequency on the AC side of the first converter station (e.g., on the rectifier side of the system). The difference between the second time and the first time may be, for example, equal to or less than about 20 milliseconds (which may correspond to an AC frequency of 50 Hz), or equal to or less than about 16 to 17 milliseconds (which may correspond to an AC frequency of 60 Hz), and so on.

In some embodiments, the threshold voltage value may be defined as the value of the sensed DC voltage at the first time.

In some embodiments, determining at least partial recovery of the sensed DC voltage may include determining whether a time derivative of the sensed DC voltage is equal to or greater than a threshold voltage time derivative value. Determining recovery of the at least partial recovery of the sensed DC voltage may also include determining that the at least partial recovery of the sensed DC voltage has occurred based on determining that a time derivative of the sensed DC voltage is equal to or greater than a threshold voltage time derivative value. Herein, the "derivative value" may indicate a rate of change of the sensed DC voltage over time. For example, a positive derivative value (which may exceed a threshold voltage time derivative value) may indicate that the sensed DC voltage is increasing, at least temporarily, over time. The sensed DC voltage increases at least temporarily over time and may thereby at least indicate or confirm at least partial recovery of the sensed DC voltage.

In some embodiments, the threshold current value may be proportional to the power stage number used to control the first converter station. The power stage number may also be locally available to the first converter station.

According to a second aspect of the present disclosure, a converter station for an HVDC transmission system is provided. The HVDC transmission system may comprise a DC transmission link. The converter station may be connectable to a first end of the DC transmission link. The converter station may comprise a current sensor for sensing the DC current at the first end of the DC transmission link. The converter station may comprise a voltage sensor for sensing the DC voltage at the first end of the DC transmission link. The converter station may comprise a controller. The controller may be arranged to determine whether a phase to ground fault has occurred at the second converter station connected to the second end of the DC transmission link based on the sensed DC current and the sensed DC voltage. The controller may be arranged to control the converter stations to reduce the power delivered by the converter stations to the second converter station via the DC transmission link based on the determination that a phase to ground fault has occurred.

The sensed DC current and the sensed DC voltage of the second aspect may correspond to the sensed DC current and the sensed DC voltage, respectively, described above with reference to the method according to the first aspect. Likewise, other corresponding features and results described in relation to the method of the first aspect may also be applied to the features and results of the converter station described according to the second aspect.

In some embodiments, the controller may be arranged to perform any of the methods described according to the first aspect.

In some embodiments, the converter station may comprise at least one of: line Commutated Converters (LCCs), half-bridge modular multilevel converters (HB MMCs), and full-bridge modular multilevel converters (FB MMCs).

According to a third aspect of the present disclosure, there is provided an HVDC transmission system. The HVDC transmission system may comprise a first converter station as defined above according to the second aspect. The HVDC transmission may comprise a second converter station. The HVDC transmission system may comprise a DC transmission link. The DC transmission link can connect the first converter station and the second converter station.

In some embodiments the second converter station may also be a converter station as defined above according to the second aspect.

In some embodiments, the second converter station may comprise at least one of a HB MMC and a FB MMC.

In some embodiments, the second converter station may comprise at least a HB MMC and a FB MMC. The HB MMC and the FB MMC may be connected in series.

In some embodiments, the FB MMC may be connected to the DC transmission line of the DC transmission link closer than the HB MMC. The DC transmission line may be a line on which DC power is transported from the first converter station to the second converter station.

In some embodiments, the first converter station may comprise an LCC. For example, in some embodiments the first converter station may for example comprise an LCC and the second converter station may comprise a FB MMC and an FB MMC connected in series, wherein the FB MMC is connected closer to the DC transmission line on which the power is transported than the HBMMC.

The disclosure relates to all possible combinations of features set forth in the claims. Objects and features described according to the first aspect may be combined with or substituted for objects and features described according to the second and/or third aspects, and vice versa.

Further objects and advantages of various embodiments of the present disclosure will be described below by means of exemplary embodiments.

Drawings

Exemplary embodiments will be described below with reference to the accompanying drawings, in which:

fig. 1 schematically illustrates an HVDC transmission system;

fig. 2a and 2b schematically illustrate a flow chart of an embodiment of a method according to the present disclosure;

figures 3a to 3d schematically illustrate the values of the DC current and DC voltage sensed during various faults;

fig. 4 schematically illustrates an embodiment of a converter station according to the present disclosure;

fig. 5 schematically illustrates an embodiment of an HVDC transmission system according to the present disclosure; and

fig. 6 schematically illustrates an embodiment of an HVDC transmission system according to the present disclosure.

In the drawings, like reference numerals will be used for like elements unless otherwise specified. Unless explicitly stated to the contrary, the figures only show elements that are necessary to illustrate example embodiments, while other elements may be omitted or merely implied for clarity. As illustrated in the figures, the sizes of elements and regions may be exaggerated for illustrative purposes, and thus, these elements and regions are provided to illustrate the general structure of embodiments.

Detailed Description

Referring to fig. 1, an HVDC transmission system is described next.

Fig. 1 illustrates an HVDC transmission system 100. The HVDC transmission system 100 comprises a DC transmission link 110, which DC transmission link 110 has a first end 112 and a second end 114. The first converter station 120 is connected to the DC transmission link 110 at a first end 112 and the second converter station 130 is connected to the DC transmission link 110 at a second end 114. The DC transmission link 110 can, for example, include one or more DC (transmission) lines in which DC current can flow.

The first converter station 120 comprises an HVDC converter 122, which HVDC converter 122 is connected to an AC grid 140 via a transformer 124. The transformer 124 may be part of the first converter station 120. It is envisaged that the first converter station 120 may comprise more than one HVDC converter 122, and it is also envisaged that more than one transformer 124 may be present in order to connect the first converter station 120 to the AC grid 140. In the HVDC transmission system 100 it is envisaged that the first converter station 120 acts as a rectifier, such that AC power is received from the AC grid 140 and converted to DC power by the HVDC converter(s) 122.

The second converter station 130 comprises an HVDC converter 132, which HVDC converter 132 is connected to an AC grid 142 via a transformer 134. The transformer 134 may be part of the second converter station 130. Collectively, the HVDC transmission system 100 may be operable to transfer power between an AC grid 140 and an AC grid 142. Power may be transferred, for example, from AC power grid 140 to AC power grid 142 and to AC load 144 connected to AC power grid 142.

As illustrated in fig. 1, the HVDC transmission system 100 comprises a single HVDC converter on each side. However, it is envisaged that more than one HVDC converter may be included on at least one of each side. It is also envisaged that different types of HVDC converters may be used. Examples of HVDC converters may include, for example, Current Source Converters (CSCs), such as LCCs, or Voltage Source Converters (VSCs), such as HB MMCs and FB MMCs.

As illustrated in fig. 1, the HVDC transmission system 100 is arranged in a monopolar configuration, with power returning through the ground/earth. It is also envisaged that the HVDC transmission system 100 could alternatively be arranged such that power is returned through DC transmission lines, and/or that the HVDC transmission system 100 is arranged, for example, in a bipolar configuration using more than one DC transmission line each having a high potential relative to ground. It is contemplated that such multiple DC transmission lines form part of a DC transmission link, such as DC transmission link 110.

During operation of the HVDC transmission system 100, one or more faults may occur. Examples of such faults include a DC bus fault 150, wherein for example a short (short) is created between a line of the DC transmission link 110 and ground/earth, as indicated by the broken line arrow 150. Another fault, phase to ground fault 152, may occur if, for example, a short is made between the phase line and ground/earth, as indicated by broken line arrow 152. A phase ground fault 152 may occur between the HVDC converter 132 and the transformer 134 and inside the second converter station 130. Phase to ground fault 152 may occur between a single phase and ground/earth, or even between multiple phases and ground/earth. In some cases, a phase to ground fault 152 may occur, for example, at a location between the transformer 134 and one or more phase reactors connected to the HVDC converter 132. In some cases, a phase to ground fault 152 may occur, for example, at a location between the HVDC converter 132 and one or more of such phase reactors. Another type of fault may be an external AC fault 154 between the AC line and the AC bus side or at least the ground/earth behind the transformer 134, as indicated by broken line arrow 154.

As will be described in more detail later, the HVDC converter 132 may comprise a FB MMC and a HB MMC connected in series, wherein the FB MMC is connected closer to the DC transmission line than the HB MMC, in which power is transported to the second converter station 130. When (or if) an internal phase ground fault 152 occurs, high cell voltages and high valve currents may occur, which may lead to damage by overcharging the cell capacitor voltage above the rated limit. Due to the series connection, FB MMC may for example experience at least twice its rated arm voltage (2 pu). To reduce the effects of overcharging, the DC current that ultimately charges a capacitor in, for example, a FB MMC needs to be reduced to zero (or near zero). Herein, the VSC may comprise a plurality of converter cells connected in series in each arm of the VSC. The cell may comprise, for example, a half-bridge or full-bridge configuration of switches (e.g., insulated gate bipolar transistors) and at least one DC cell capacitance.

Exemplary embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings. The drawings illustrate a presently preferred embodiment, but the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for completeness and to fully convey the scope of the disclosure to those skilled in the art.

With reference to fig. 2a and 2b, various embodiments of methods of operating converter stations in an HVDC transmission system, such as the HVDC transmission system 100 shown in fig. 1, are described next.

Fig. 2a illustrates a flow chart of a method 200 of operating a converter station 130 in an HVDC transmission system 100. In step S210, the DC current and the DC voltage are sensed at the first end 112 of the DC transmission link 100. The sensed DC current (i _ DC) and the sensed DC voltage (u _ DC) are then passed to step S220, where in step S220 it is determined whether a phase to ground fault 152 has occurred at the second converter station 130 based on the sensed DC current and the sensed DC voltage. If it is determined in step S220 that a phase to ground fault 152 has occurred (result 201), the method can proceed to step S230 where the power delivered by the first converter station 120 (via the DC transmission link 110) to the second converter station 130 is reduced in step S230.

If it is determined in step S220 that a phase to ground fault 152 has not occurred (result 202), the method 200 may continue, for example, by returning again to step S210. If step S230 is reached and the power is reduced, the method 200 may, for example, stop, return to step S210, or continue to one or more other steps (not shown in FIG. 2 a).

Fig. 2b illustrates step S220 in one embodiment of the method 200 in more detail. In sub-step S222, the sensed DC current (i _ DC) is received and compared to a threshold current value (i _ DC _ th). If it is determined that the sensed DC current is equal to or greater than the threshold current value (i _ DC ≧ i _ DC _ th), step S220 may proceed to substep S224. In sub-step S224, the sensed DC voltage (u _ DC) is received and it is determined whether at least partial recovery of the sensed DC voltage has occurred based on the sensed DC voltage. If it is determined in sub-step S224 that at least partial recovery of the sensed DC voltage has occurred, step S220 may exit with result 201. Exiting with result 201 may correspond to determining that a phase grounding fault has occurred.

If it is determined in sub-step S224 that at least partial recovery of the sensed DC voltage has not occurred, step S220 may exit, for example, with result 202. Likewise, if it is determined in sub-step S222 that the sensed DC current is below the threshold current value (i _ DC < i _ DC _ th), step S220 may exit with result 202. Exiting with result 202 may correspond to determining that no phase ground fault has occurred.

In one embodiment of the method 200, determining at least partial recovery of the sensed DC voltage (as performed in, for example, sub-step S224) may include comparing the sensed DC voltage (u _ DC) to a threshold voltage value (u _ DC _ th). If the sensed DC voltage is equal to or greater than a threshold voltage value (u _ DC ≧ u _ DC _ th), it may be determined that at least partial recovery of the sensed DC voltage has occurred. If the sensed DC voltage is below the threshold voltage value, it may be determined that at least partial recovery of the sensed DC voltage has not (yet) occurred. In some embodiments of the method 200, if it is determined that the sensed DC voltage has not recovered, the sensed DC voltage may be checked again during a limited period of time. If the sensed DC voltage has not reached or exceeded the threshold voltage value after the limited period of time has elapsed, for example, it may be determined that at least partial recovery of the sensed DC voltage has not occurred. This may indicate that there is no phase-to-ground fault.

With reference to fig. 3a and 3b, an example embodiment of the method 200 during a phase-to-ground fault will now be described in more detail.

Fig. 3a schematically illustrates a plot 300 of time values of DC current (i _ DC, solid line 310) and DC voltage (u _ DC, dashed line 320) as a function of time t during and before a phase to ground fault occurring at the second converter station and as sensed at the first converter station. The units on the vertical axis may for example correspond to units (pu) defined using a per unit system, wherein the current is given as a fraction of the defined base current and the voltage is given as a fraction of the defined base voltage.

The sensed DC current 310 and the sensed DC voltage 320 are approximately constant over time before a phase-to-ground fault occurs. When a phase-to-ground fault occurs, the sensed DC current 310 begins to increase and assumes a maximum current value at time t _ 1. At time t _1, the value of the sensed DC voltage 320 is recorded and taken as the threshold voltage value (u _ DC _ th). In some embodiments, time t _1 may be defined differently, for example, as when sensed DC current 310 has risen to or above a predefined threshold current value (not shown in fig. 3 a).

The method may then anticipate that the sensed DC voltage 320 recovers, or at least partially recovers, before time t _2, which occurs after time t _ 1. The time t _2 may be selected, for example, to be several tens of milliseconds after the time t _ 1. As can be seen from fig. 3a, the sensed DC voltage 320 returns to the threshold voltage value at time t _ f, which occurs before time t _ 2. Thus, at time t _ f, the method determines that at least partial recovery of the sensed DC voltage 320 has occurred and reduces the power delivered from the first converter station to the second converter station. The DC current 310 is reduced to zero and potentially dangerous overcharging of e.g. a capacitor in the second converter station can be avoided. Since the method may be completely dependent on parameters locally available at the first converter station, such as the sensed DC current 310 and the sensed DC voltage 320, no communication channel between the first converter station and the second converter station is required. As previously described herein, this may increase the reliability of the HVDC transmission system and also reduce its complexity and cost.

Similar to fig. 3a, fig. 3b schematically illustrates a plot 301 of time values of DC current (i _ DC, solid line 311) and DC voltage (u _ DC, dashed line 321) as a function of time t during and before a phase to ground fault occurring at the second converter station and as sensed at the first converter station.

The sensed DC current 311 and the sensed DC voltage 321 are approximately constant over time before a phase-to-ground fault occurs. When a phase-to-ground fault occurs, the sensed DC current 311 begins to increase and reaches a threshold current value (i _ DC _ th) at time t _ 1. The threshold current value is a predefined value and corresponds to, for example, 1.05 pu. One "pu" (in terms of current) may for example be defined when the DC current 311 equals the DC current progression, i.e. the amount of current expected to be delivered to the converter station at a certain time. Thus, in this example, a sensed current value of 1.05pu may indicate that the current has increased by 5% from its expected value.

The time t _1 at which the sensed DC current 311 reaches the threshold current value is taken as an indication that a possible fault has occurred. The method then "waits" until a later time t _2, and at least partial recovery of the sensed DC voltage 321 is expected to occur before time t _ 2. In contrast to the embodiment of the method described with reference to fig. 3a, in the embodiment of the method described with reference to fig. 3b, the threshold voltage value is defined as 1.1pu (i.e. where the sensed DC voltage 321 rises above 10% of the DC voltage level). As can be seen in fig. 3b, this occurs at time t _ f. At time t _ f, it is determined that at least partial recovery of the sensed DC voltage 321 has occurred and the first converter station is instructed to reduce (via the HVDC transmission link) the power delivered to the second converter station. Of course, it is contemplated to use other threshold voltage values than 1.1pu, such as, for example, 0.9 pu.

In some embodiments of method 200, instead of or in addition to waiting for the sensed DC voltage to reach or exceed a particular threshold voltage value, method 200 may anticipate the sensed DC voltage to begin increasing at a particular rate. This increase at a particular rate may indicate that at least partial recovery of the sensed DC voltage has begun. A "particular rate of increase" may correspond to, for example, the time derivative of the sensed DC voltage being equal to or greater than a threshold voltage time derivative value. The time derivative of the sensed DC voltage may be obtained, for example, by calculating the change (or difference) in voltage over time between two sensed DC voltage values and, for example, by dividing the difference by the time difference between the times at which the two sensed DC voltage values were obtained. Other methods of obtaining the time derivative (or "rate of change") are also contemplated. The threshold voltage time derivative value may, for example, correspond to 0.02pu and/or be calculated to be proportional to the number of power stages supplied to the first converter station divided by the rated power of the first converter station. In some embodiments, the threshold voltage time derivative value may be, for example, zero, such that the required sensed DC voltage is increasing. Using the time derivative of the sensed DC voltage may allow for a more accurate and/or faster detection of a phase ground fault and may also allow for distinguishing such a phase ground fault from, for example, a DC bus fault and/or an external AC fault. Of course, it is contemplated that other threshold voltage time derivative values other than 0.02pu may be used.

Also, in some embodiments of method 200, instead of or in addition to waiting for the sensed DC current to reach or exceed a particular threshold current value before indicating a latent fault, method 200 may anticipate the sensed DC current to begin to increase at a particular rate. This "growth rate" or time derivative of the sensed DC current may be obtained as described above for the sensed DC voltage but alternatively may use two or more sensed DC current values and compare the calculated current time derivative to a threshold current time derivative value. Using the time derivative of the sensed DC voltage may allow for a more accurate and/or faster detection of a phase ground fault and may also allow for distinguishing such a phase ground fault from, for example, a DC bus fault and/or an external AC fault.

Referring to fig. 3c and 3d, the method 200 will now be described in more detail when experiencing other faults than a phase-to-ground fault.

Fig. 3c schematically illustrates a plot 302 of time values of DC current (i _ DC, solid line 312) and DC voltage (u _ DC, dashed line 322) as a function of time t during and before an external AC fault occurring at the second converter station and as sensed at the first converter station.

Before an external AC fault occurs, the sensed DC current 312 and the sensed DC voltage 322 are approximately constant over time. When an external AC fault occurs, the sensed DC voltage 322 begins to rise and the sensed DC current 312 begins to fall. Method 200 may distinguish such an external AC fault because there is no increase in sensed DC current 312.

Fig. 3d schematically illustrates a plot 303 of time values of DC current (i _ DC, solid line 313) and DC voltage (u _ DC, dashed line 323) as a function of time t and as sensed at the first converter station during and before a DC bus fault occurring at the HVDC transmission link.

The sensed DC current 313 and the sensed DC voltage 323 are approximately constant before a DC bus fault occurs. When a DC bus fault occurs, the sensed DC current 313 begins to rise and the sensed DC voltage 323 begins to fall. There is no recovery of the sensed DC voltage 323 even though the sensed DC current 313 may exceed the threshold current value. Alternatively, the sensed DC voltage 323 is near zero, and the method 200 may also distinguish between such DC bus faults.

As illustrated with reference to fig. 3a to 3d, the method 200 according to the present disclosure may allow a correct determination of the occurrence of a phase ground fault at the second converter station locally at the first converter station in a non-communicative manner (i.e. without the need for a communication channel). The method 200 may also correctly distinguish such phase-to-ground faults from, for example, external AC faults and DC bus faults. When it is determined that a phase to ground fault has occurred at the second converter station, the method may correspondingly reduce the power delivered from the first converter station to the second converter station so that e.g. overcharging of the cell capacitor in the second converter station is avoided.

In some embodiments of method 200, the sensed DC current values and DC voltage values may alternatively be filtered before being used. Using filtered values may, for example, help to avoid erroneously determining the occurrence of a phase-to-ground fault due to, for example, noise.

With reference to fig. 4, an embodiment of a converter station according to the present disclosure will now be described in more detail.

Fig. 4 schematically illustrates a converter station 400. The converter station 400 may be comprised in an HVDC transmission system comprising a DC transmission link 410. The converter station 400 can be connected to a first end 412 of the DC transmission link 410. The converter station 400 is also connectable to an AC power grid 414 such that the converter station 400 is operable to convert power between the AC power grid 414 and the DC transmission link 410. At the other end of the DC transmission link 410, a second converter station (not shown in this figure) may be connected to receive power from the converter station 400, convert the received power back into AC power, and transmit the AC power to a second AC grid (not shown). The converter station 400, the DC transmission link 410 and the second converter station may form part of an HVDC transmission system.

The converter station 400 comprises at least one HVDC power converter 420, which at least one HVDC power converter 420 performs conversion between AC and DC power. The converter station 400 is connected to the AC grid 414 via a transformer arrangement 422. The transformer arrangement 422 may be part of the converter station 400, but may also be considered as a separate part.

The converter station 400 comprises a current sensor 430 and a voltage sensor 432. The current sensor 430 is arranged to sense the DC current at or near the first end 412 of the DC transmission link 410. Here, "close" means that the sensed value is sensed locally to the converter station 400, and not, for example, farther along the DC transmission link 410 or at another converter station connected to the DC transmission link 410. In other words, the current and voltage may be measured at a location accessible to the converter station (i.e. no separate communication channel is required) (and the sensors may be located at such a location). With respect to the sensing of current and voltage, the word "end" of the DC transmission link is not defined herein as the very end of the DC transmission link, but as a side of the HVDC transmission system.

Likewise, the voltage sensor 432 is arranged to sense the DC voltage at or near the first end 412 of the DC transmission link 410. Although illustrated as separate components/objects, it is contemplated that current sensor 430 and voltage sensor 432 may form part of the same component.

The converter station 400 further comprises a controller 440, which controller 440 receives the sensed DC current and the sensed DC voltage from the current sensor 430 and the voltage sensor 432, respectively. The sensed values may be communicated to controller 440, for example, using wires or over one or more wireless links. It should be repeated for clarity that such transmission of values is still considered local to the converter station 400, and that the sensors 430 and 432 may for example be located within the same construction/housing as, for example, the HVDC converter 420 or similar locations. In other words, the distance between the converter station 400 and the second converter station is significantly larger than the distance between each of the sensors 430 and 432, for example, and the HVDC converter 420, for example.

The converter station 400 is arranged to determine whether a phase to ground fault has occurred at the second converter station connected to the second end of the DC transmission link based on the sensed DC current and the sensed DC voltage (as described above with reference to fig. 3a to 3 d). The converter station 400 is further arranged to: if a phase to ground fault is detected/determined to have occurred, the converter station 400 is controlled to reduce the power delivered by the converter station 400 to the second converter station via the DC transmission link 410. This may be achieved, for example, by the controller 440 providing one or more control signals to the HVDC converter 420. If the HVDC converter is a current source converter, such as an LCC, the controller 440 may reduce the delivered power, for example by providing an adjusted (increased) firing angle (firinggle) to the HVDC converter 420. The firing angle may be defined as the time such LCC delays/advances the turn-on of its respective thyristor.

In some embodiments, the controller 440 may also be configured to perform the method 200, wherein various embodiments of the method 200 have been previously described herein.

The HVDC converter 420 may for example be an LCC, HB MMC or FB MMC. The converter station 400 may comprise more than one HVDC converter, and may also comprise more than one type of HVDC converter, and the exact number and type may be adjusted depending on the requirements at the converter station 400 (in terms of power delivered, voltage available, etc.).

With reference to fig. 5 and 6, an embodiment of an HVDC transmission system according to the present disclosure will now be described in more detail.

Fig. 5 schematically illustrates an embodiment of an HVDC transmission system 500. The HVDC transmission system 500 comprises an HVDC transmission link 510 (which may be, for example, an HVDC power line), a first converter station 520 and a second converter station 550. At least the first converter station 520 is a converter station as previously described herein, e.g. the converter station 400 described with reference to fig. 4. An HVDC transmission link 510 connects the first converter station 520 and the second converter station 550. The HVDC transmission system 500 may also be connected to AC grids 560 and 562, such that AC power on the AC grid 560 may be converted to DC power by the first converter station 520, transmitted to the second converter station 550 via the DC transmission link 510, and then converted back to AC power by the second converter station 550 and output on the AC grid 562.

The first converter station 520 is connected at the first end 512 of the DC transmission link 510 and the first converter station 520 includes a current sensor 530 for sensing the DC current at the first end 512 of the DC transmission link 510. The first converter station 520 further comprises a voltage sensor 532 for sensing the DC voltage of the DC transmission link 510 at the first end 512. The first converter station 520 further comprises a controller 540 arranged to determine the occurrence of a phase to ground fault at the second converter station 550 based on the sensed DC current and the sensed DC voltage and to reduce the power delivered by the first converter station 520 if it is determined that such a phase to ground fault has occurred. For more details of the operation of the first converter station 520, reference is made to the method 200 and the various embodiments of the converter station 400 described previously herein.

Fig. 6 schematically illustrates an embodiment of an HVDC transmission system 600. The HVDC transmission system 600 is arranged in a bipolar configuration and comprises an HVDC transmission link comprising a first DC transmission line 610 and a second DC transmission line 611. The HVDC transmission system 600 comprises a first converter station 620 and a second converter station 650. The converter stations 620 and 650 are connected to each other via a first DC transmission line 610 and a second DC transmission line 612.

The first converter station 620 comprises a plurality of Current Source Converters (CSCs) 622 connected in series. The CSC 622 may be, for example, an LCC. In the example illustrated in fig. 6, two CSCs are connected in series between ground/earth and each pole (e.g., each of the first DC transmission line 610 and the second DC transmission line 611). As previously described, the first converter station 620 also includes a current sensor 630, a voltage sensor 632, and a controller 640. The functionality of the current sensor 630, the voltage sensor 632, and the controller 640 is the same as previously described herein with reference to fig. 4. When it is determined that a phase to ground fault has occurred at the second converter station 650, the controller 640 can control the CSC 622 to reduce the power delivered from the first converter station 620 to the second converter station 650, i.e. by reducing the DC current on the first and second DC transmission links 610 and 611, without any communication channel between the first converter station 620 and the second converter station 650. A controller 640 may control all CSCs 622. It is also contemplated that a second controller (not shown), a second current sensor (not shown) and a second voltage sensor (not shown) are used to control, for example, a CSC connected to the DC transmission line 611. Such second current and voltage sensors may, for example, sense current and voltage at the second DC transmission line 611.

The second converter station 650 may comprise a plurality of Voltage Source Converters (VSCs) in different configurations. Between the ground/earth and each of the first DC transmission line 610 and the second DC transmission line 611, a mixture of HB MMC 654 and FB MMC652 may be serially linked. In the example illustrated in fig. 6, one FB MMC652 and one HB MMC 654 are connected between ground/earth and each of the first DC transmission line 610 and the second DC transmission line 611, but it is also conceivable that more than one VSC of each type may be used and connected in series.

In a (hybrid) HVDC transmission system 600, the series connection of two VSCs 652 and 654 may allow super HVDC to be achieved. The use of VSCs at the receiving (inverting) end of the HVDC transmission system 600 can reduce commutation failure problems, such as might occur if LCCs were used at both ends of the system. Due to the series connection, during a phase to ground fault at the second converter station 650, the VSC 652 closest to the respective first and second DC transmission line 610, 611 at the second converter station 650 may be subjected to very high voltage stress. Thus, the method 200 and the converter station 400 of the present disclosure may be adapted to detect and take action on such phase-to-ground faults in the hybrid HVDC transmission system 600.

The person skilled in the art realizes that the present disclosure by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

In addition, variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.