阅读说明：本技术 电路断路器中的健康调查 (Health investigation in circuit breakers ) 是由 A·库玛 S·N·R·帕慕拉帕蒂 A·温伯格 P·卡尔森 Y·彼得森 于 2018-03-21 设计创作，主要内容包括：一种用于具有反向阻断状态、正向导电状态和故障状态的第一单向导电元件(MBD1)的健康调查装置(22)包括：与元件(MBD1)并联的分压电路(26),该电路(26)包括第一块(40),该第一块(40)包括与第二块(42)串联连接的第一部件(R1),该第二块(42)包括第二部件(R2),这些部件彼此之间具有将反向阻断状态电压分压成可以由信号评估电路处理的电平的关系。块(40,42)之间的结点形成提供能够表示所有状态的健康信号(Vmon)的健康信号输出,并且块(42)包括与第一(MBDi)相比具有相反取向的第二单向导电元件(D2)用于在第一元件处于正向导电状态时从健康信号中去除分压。(A health investigation device (22) for a first unidirectional conductive element (MBD1) having a reverse blocking state, a forward conducting state, and a fault state comprising: a voltage divider circuit (26) connected in parallel with the element (MBD1), the circuit (26) comprising a first block (40), the first block (40) comprising a first block (R1) connected in series with a second block (42), the second block (42) comprising a second block (R2), the blocks having a relationship to each other to divide the reverse blocking state voltage to levels that can be processed by the signal evaluation circuit. The junction between the blocks (40, 42) forms a health signal output providing a health signal (Vmon) representative of all states, and the block (42) includes a second unidirectional conductive element (D2) having an opposite orientation compared to the first (MBDi) for removing a partial voltage from the health signal when the first element is in a forward conductive state.)

1. A health investigation device (22) for a first self-commutated unidirectional conducting element (MBD1) acting as a main circuit breaker element of a circuit breaker (20) connected in series with a direct current power transmission medium (18), said element having a reverse blocking state (S1) at a first voltage level, a forward conducting state (S2) at a second voltage level, and a fault state (S3) at a third voltage level, wherein the absolute value of the second voltage level is higher than the absolute value of the third voltage level, and the absolute value of the first voltage level is more than ten times the absolute value of the second voltage level, the health investigation device (22) comprising a voltage divider circuit (26) connected in parallel with the first unidirectional conducting element (MBD1), the voltage divider circuit (26) comprising:

a first voltage processing block (40) connected in series with a second voltage processing block (42), the first voltage processing block (40) comprising a first voltage dividing component (R1) and the second voltage processing block (42) comprising a second voltage dividing component (R2), the first and second voltage dividing components (R1, R2) having a relationship with each other arranged to divide the voltage of the reverse blocking state (S1) to a level that can be processed by a signal evaluation circuit (28), wherein a junction between the first and second voltage processing blocks (40, 42) forms a healthy signal output on which a healthy signal (Vmon) is provided that can represent all states of the first unidirectional conductive element, wherein the second voltage processing block (42) comprises a second unidirectional conductive element (D2) having an opposite orientation compared to the first unidirectional conductive element (MBD1) to place the first unidirectional conductive element in the forward conductive state (S2) ) The partial pressures of the first and second voltage-dividing components are removed from the health signal.

2. The health investigation device (22) of claim 1, wherein the second voltage dividing member (R2) is connected in series with the second unidirectional conductive element (D2).

3. The health investigation device (22) of claim 2, wherein the second voltage processing block (42) comprises a further reactive component (C2) connected in parallel with the second voltage dividing element (R2) and the second unidirectional conductive element (D2).

4. The health investigation device (22) of claim 3, wherein the reactive component (C2) is a capacitor.

5. The health investigation device (22) of claim 3 or 4, wherein the first voltage processing block (40) comprises a reactive component (C1) of the same type as the reactive component (C2) in the second voltage processing block (42) connected in parallel with the first voltage dividing component (R1).

6. The health investigation device (22) of any preceding claim 4, further comprising a signal evaluation circuit (28) configured to determine a state of the first unidirectional conductive element (MDB1) based on the health indication signal (Vmon).

7. The health investigation device (22) as claimed in claim 6, wherein the signal evaluation circuit (28) is configured to determine the state of the first unidirectional conductive element (MDB1) by comparing the health indication signal (Vmon) with three different thresholds, each associated with a corresponding state, and to indicate a state crossing the corresponding threshold.

8. The health investigation device (22) as claimed in claim 6, wherein the signal evaluation circuit (28) is configured to encode a voltage level of the health indication signal (Vmon) as a pulse train having a pulse repetition rate corresponding to the voltage level of the health indication signal (Vmon) to indicate the state.

9. The health investigation device (22) as claimed in any of the previous claims, further comprising a power supply circuit (24) for supplying power to the circuit of the device, the power supply circuit (24) comprising a direct current/direct current DC/DC converter (34) and a first and a second voltage dividing block (30, 32), the first voltage dividing block (30) and the second voltage dividing block (32) being connected in series in a first string, the first string being connected in parallel with the first unidirectional conductive element (MBD1), wherein a first junction between the first and second voltage dividing blocks (30, 32) is connected to a first input of the DC/DC converter (34) and the first and second voltage dividing blocks (30, 32) have a voltage divider between each other arranged to provide a voltage at the first junction as the reverse blocking state when the first unidirectional conductive element (MBD1) is in the reverse blocking state to be capable of being divided by a DC/DC converter (34) A voltage of a processed level, wherein the power circuit (24) is further configured to provide a voltage corresponding to the voltage of the forward conduction state at a level that can be processed by the DC/DC converter (34) when the first unidirectional conductive element (MBD1) is in the forward conduction state.

10. The health investigation device (22) according to claim 9, wherein the first and second blocks have a relation to each other when the first unidirectional conductive element (MBD1) is in the forward conducting state arranged to provide the voltage at the first junction corresponding to the voltage of the forward conducting state at a level that can be handled by the DC/DC converter (34).

11. The health investigation device (22) of claim 9, further comprising a second string having a third voltage division block (36) connected in series with a fourth voltage division block (38), the second string being connected in parallel with the first unidirectional current conducting element (MBD1), wherein a second junction between the third voltage division block and the fourth voltage division block is connected to a first input of the DC/DC converter (34) and the third voltage division block and the fourth voltage division block have a relation to each other arranged to provide the voltage at the second junction at a level processable by the DC/DC converter (34) corresponding to the voltage of the forward conducting state when the first unidirectional conducting element (MBD1) is in the forward conducting state.

12. The health investigation device (22) according to any of claims 9-11, further comprising a selection block (35) connected between the first node and the first input of the DC/DC converter (34), the selection block being configured to select whether the voltage corresponding to the voltage of the forward conducting state or the divided voltage of the reverse blocking state is to be supplied to the DC/DC converter (34).

13. The health survey device of claim 12 wherein the selection block is further configured to reverse the polarity of one of the voltages.

14. A circuit breaker (20) connected in series with a direct current power transmission medium (18), the circuit breaker comprising a unidirectional conducting element (MBD1) acting as a main circuit breaker element and a health investigation device (22) according to any of the preceding claims.

15. A power transmission system comprising a transmission medium and a series connection of circuit breakers (20) according to claim 14.

Technical Field

The present invention relates generally to investigating the health of unidirectional conductive elements in circuit breakers. More particularly, the invention relates to a health investigation device for a first unidirectional conducting element acting as a main circuit breaker element of a circuit breaker, a circuit breaker comprising such a health investigation device and a transmission system comprising such a circuit breaker.

Background

In power transmission systems, such as High Voltage Direct Current (HVDC) power transmission, circuit breakers are often required in order to disconnect a wire or cable during a pole fault, such as a pole-to-ground fault or reverse polarity. This may be done to protect the main converter valves from system induced reverse polarity. The DC circuit breaker may then comprise a main breaker consisting of a plurality of series-connected semiconductor components connected in parallel with surge arresters. There may also be an electronic load reversing switch and a mechanical isolator connected in parallel with the main circuit breaker.

Main breakers are often (especially when bidirectional current conduction is envisaged) realized by a combination of forced commutation semiconductor switches, such as transistors with anti-parallel diodes.

In some cases it is interesting to replace the above described forced commutation switches with self-commutating semiconductors or unidirectional conducting elements, such as diodes. This may be advantageous when only one current conduction direction is actually envisaged in the power transmission system, in which case no logic is required to control the main breaker to interrupt the current.

At the same time, however, it is of interest to monitor the health of such self-commutating semiconductor or unidirectional conducting elements, i.e., to determine whether such self-commutating semiconductor or unidirectional conducting elements are functioning properly.

Some solutions of monitoring diodes are known.

US 2004/0125518 discloses monitoring a diode acting as a reverse current protection device for a fuel cell. The voltage across the diode is measured and reversed. The reverse voltage is then compared to a threshold value to determine that the diode is good if the threshold value is crossed.

CH 513539 discloses a semiconductor valve monitoring device for high voltage applications, wherein the valve may consist of a diode or a thyristor as valve element. The health of the thyristors of the valve is determined by comparing the voltage across the entire valve with the voltage across the first valve element in the amplifier.

The diodes used in high voltage systems will basically have two health states; a forward biased state and a reverse biased state. The forward bias state typically involves a rather low voltage across the diode, such as in the range of 0.8-1.35V, while the reverse bias voltage will be considerably higher, such as in the range of 3-5 kV. In addition, the voltage of the failing diode may be about zero.

Due to the differences between the voltage levels of the different states and in particular between the healthy states, it is still difficult to distinguish between the different states and in particular between the fault state and the healthy forward biased state. Thus, it may be difficult to determine the health condition.

The present invention addresses the problem of simplifying the distinction between different states of a unidirectional conductive element in order to improve the determination of the health of the semiconductor.

Disclosure of Invention

It is therefore an object of the present invention to improve the determination of the health of a unidirectional conductive element.

According to a first aspect, the object is achieved by a health investigation device or a first self-commutated unidirectional conducting element acting as a main circuit breaker element of a circuit breaker connected in series with a direct current transmission medium, the element having a reverse blocking state at a first voltage level, a forward conducting state at a second voltage level and a fault state at a third voltage level, wherein the absolute value of the second voltage level is higher than the absolute value of the third voltage level and the absolute value of the first voltage level is more than ten times the absolute value of the second voltage level. The health investigation device includes a voltage divider circuit connected in parallel with a first unidirectional conductive element, the voltage divider circuit including:

a first voltage processing module connected in series with a second voltage processing block, the first voltage processing block comprising a first voltage dividing component and the second voltage processing block comprising a second voltage dividing component, the first and second voltage dividing components having a level between each other arranged to divide a voltage of a reverse blocking state into levels that can be processed by the signal evaluation circuit, wherein a junction between the first voltage processing block and the second voltage processing block forms a health signal output, a health signal representative of all states of the first unidirectional conductive element is provided on the health signal output, wherein the second voltage handling block includes a second unidirectional conductive element having an opposite orientation as compared to the first unidirectional conductive element, so that the divided voltages of the first and second voltage-dividing parts are removed from the health signal when the first unidirectional conductive element is in the forward conductive state.

According to a second aspect, the object is also achieved by a circuit breaker comprising a unidirectional conducting element acting as a main circuit breaker element and a health investigation device according to the first aspect.

According to a third aspect, the object is achieved by a power transmission system comprising a transmission medium and series-connected circuit breakers according to the second aspect.

The unidirectionally conducting element may be a unidirectionally conducting semiconductor such as a diode or a thyristor. A unidirectional conducting element may also be considered a self-commutating element or a self-commutating semiconductor, since it is turned off by itself, rather than by receiving a control signal. Since the unidirectional conductive element can be deactivated by being reverse biased, it can also be considered a reverse biased deactivation element. In the case of a diode, a unidirectional conducting element may also be considered a forward biased conducting element, since it may conduct by being forward biased. The polarity of the first voltage level may be opposite to the polarity of the second voltage level. The absolute value of the first voltage level may additionally be greater than one thousand times and advantageously greater than two thousand times the absolute value of the second voltage level.

The invention according to the above aspects has many advantages. The invention allows to safely detect the different states of the unidirectional conducting element in a simple, effective and reliable way, even if the voltage being monitored may be at a very low level and at a high level.

The first and second voltage processing blocks may be implemented using only passive components and thus a health signal may be obtained without using any power supply, which may be difficult to implement locally at the circuit breaker.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Drawings

The invention will now be described with reference to the accompanying drawings, in which

Figure 1 schematically shows a simple HVDC system comprising a power transmission medium in the form of a wire or cable connected to a circuit breaker,

figure 2 schematically shows a circuit breaker comprising a main breaker consisting of a plurality of series-connected diodes and a health investigation device connected in parallel with one of the diodes,

fig. 3 schematically shows a health investigation device and one of the diodes, wherein the device comprises a power supply circuit, a voltage dividing circuit and a signal evaluation circuit,

figure 4 schematically shows a diode together with a voltage divider circuit and a first variant of a power supply circuit,

figure 5 schematically shows one implementation of a voltage divider circuit,

figure 6 schematically shows one implementation of a signal evaluation circuit,

figure 7 shows an example implementation of a first variant of the power supply circuit,

FIG. 8 schematically illustrates one manner in which a health assessment signal may be encoded to indicate various states of a diode according to another implementation of a voltage divider circuit, an

Fig. 9 schematically shows a second variation of the voltage dividing circuit together with the power supply circuit.

Detailed Description

Hereinafter, a detailed description will be given of preferred embodiments of the present invention.

The present invention relates to a health investigation apparatus for a unidirectional conductive element and a circuit breaker connected in series with a direct current transmission medium. The invention also relates to a direct current transmission system comprising such a transmission medium and a circuit breaker.

Fig. 1 shows a variant of a High Voltage Direct Current (HVDC) transmission system.

The system in fig. 1 is a point-to-point system for connection between two Alternating Current (AC) power transmission systems. For this reason, an HVDC system comprises: first and second converter stations 10 and 12, wherein the first converter station 10 comprises a first transformer T1. The first converter station 10 further comprises a first converter 14 for converting between AC and DC, which converter 14 thus comprises an AC side connected to transformer T1 and a DC side connected to a first reactor L1. Thus, a first transformer T1 connects the first converter 14 to a first AC transmission system (not shown). The first converter 14 is connected to a second converter 16 of the second converter station 12 via a DC transmission medium 18, which may be a wire or a cable. As previously mentioned, the first converter 10 may here be connected to a first end of the transmission medium via a first reactor L1 and the second converter may be connected to a second end of the transmission medium 18 via a second reactor L2. The second converter 16 also converts between AC and DC and may be an inverter. The second converter station 12 may further comprise a second transformer T2 connecting the second converter 16 to a second AC power transmission system (not shown).

Converters 14 and 16 may be any type of converter such as a line commutated Current Source Converter (CSC) or a forced commutated Voltage Source Converter (VSC). The converter may more particularly comprise a plurality of converter valves. The voltage source converter may be a two-level voltage source converter or a multi-level voltage source converter employing cells (cells). Finally, the circuit breaker 20 is also connected in series with the transmission medium. Although only one circuit breaker is shown, it should be appreciated that there may be one such circuit breaker in the vicinity of the inverters, for example in the vicinity of each inverter. Thus, there may be devices located on opposite sides of the transmission medium 18.

The HVDC system in fig. 1 is a monopole system. However, it should be appreciated that the system may also be a bipolar system.

As another alternative, the HVDC system may be a multi-terminal HVDC system, such as an HVDC system comprising a plurality of converters connected to a DC grid, wherein the converters convert between AC and DC. In this case, the wires of such a grid may each comprise one, two or more circuit breakers.

Fig. 2 illustrates one manner in which the circuit breaker 20 may be implemented. The circuit breaker 20 may be composed of one or more series-connected blocks, of which four are shown in fig. 2. In such a block, there may be a set of parallel branches. In this example, the group consists of two parallel branches. There is a first main breaker branch and a second surge arrester branch. Herein, each surge arrester branch comprises a surge arrester and each main breaker branch comprises a self-commutating unidirectional current conducting element or a self-commutating semiconductor element in the form of a diode. Thus, there are first, second, third and fourth diodes MBD1, MBD2, MBD3 and MBD4 connected in series for forming a main circuit breaker.

It should be appreciated that the illustrated circuit breaker 20 is only one possible implementation. There are many different implementations, such as by each module including a mechanical isolator and/or each module including a snubber circuit in parallel with the main breaker leg and the surge arrester leg. The circuit breaker may not be modular but instead provide three parallel strings of diodes, surge arresters and load-reversing switches with mechanical isolators. Another possible implementation is to connect a load commutation branch comprising an electronic load commutation switch and a mechanical isolator in parallel with the main breaker branch and the surge arrester branch.

In all these variants, however, the main breaker is realized using a self-commutating semiconductor or a unidirectional current conducting element or semiconductor, in which case the unidirectional current conducting element or semiconductor is also a diode. For this reason, the circuit breaker 20 may also be considered as a diode valve.

In the figure, the health investigation device 22 is also shown connected in parallel with a first diode MBD 1. A health investigation device 22 is provided to investigate the health of the diode: it should be appreciated that even though only one health survey device is shown, all of the diodes MBD1, MBD2, MBD3, and MBD4 of the main circuit breaker 20 may be connected to such a health survey device 22.

Fig. 3 schematically illustrates one way of implementing the health survey apparatus 22. As can be seen, the health survey device 22 includes: a power supply circuit 24 connected in parallel with the diode MBD1, and a voltage divider circuit 26 connected in parallel with the power supply circuit 24. There is also a signal evaluation circuit 28 connected to both the power supply circuit 24 and the voltage divider circuit 26. The power supply circuit 24 provides power to the circuitry that operates the device, such as the signal evaluation circuit 28 and perhaps the voltage divider circuit 26, which is derived from the voltage across the diode MBD 1. The voltage divider circuit 26 provides the monitor voltage Vmon to the signal evaluation circuit 28 for evaluating the health of the diode MBD 1. Accordingly, the signal evaluation circuit 28 determines the state of the first unidirectional conductive element MDB1 based on the health indication signal Vmon.

It should be realized here that the signal evaluation circuit 28 may be provided with its own local power supply, in which case the power supply circuit 24 may be omitted. The signal evaluation circuit 28 may also be powered by the voltage divider circuit 26 itself under certain conditions, and in this case, the power supply 24 may also be omitted.

Fig. 4 shows a first implementation of the voltage divider circuit 26 together with the power supply circuit 24. The power supply circuit 24 includes: a first DC/DC converter 34 and first and second voltage dividing blocks 30 and 32, wherein the first voltage dividing block 30 and the second voltage dividing block 32 are connected in series in a first string, wherein the first string with the two blocks is further connected in parallel with a diode MBD 1. Furthermore, the first voltage-dividing block 30 is connected to the cathode C of the diode MBD1 and the second voltage-dividing block 32 is connected to the anode a of the diode MBD 1. There is also a third voltage dividing block 36 connected in series with the fourth voltage dividing block 38, wherein these two blocks are also connected in parallel with the diode MBD1, wherein the third voltage dividing block 36 is connected to the cathode C of the diode MBD1 and the fourth voltage dividing block 38 is connected to the anode a of the diode MBD 1. A first junction between the first and second voltage division blocks 30 and 32 is connected to a first input of a first DC/DC converter 34. Also, a second junction between the third and fourth voltage dividing blocks 36 and 38 is connected to a first input of the first DC/DC converter 34. In this first modification of the power supply circuit 24, the first node and the second node are connected to the first input terminal of the first DC/DC converter 34 via the selection block 35. Thus, there is a selection block 35 connected between the first junction point, the second junction point and the first input of the first DC/DC converter 34. The junction between the anode a of the diode MBD1, the second voltage division block 32 and the fourth voltage division block 38 is also connected to the second input terminal of the first DC/DC converter 34. The first and second voltage dividing blocks 30 and 32 may be considered to form a first voltage divider, and the third and fourth voltage dividing blocks 36 and 38 may be considered to form a second voltage divider. It should here be realised that the function of the selection block 35 is optional and that its function may be included in the first and/or second voltage divider.

The voltage divider circuit 26 is connected in parallel with a first unidirectional conductive element (MBD 1). The voltage divider circuit 26 comprises a first voltage processing block 40 connected in series with a second voltage processing block 42 and the series connection is likewise connected in parallel with a diode MBD1, wherein the first voltage processing block 40 is connected to the cathode C of the diode MBD1 and the second voltage processing block 42 is connected to the anode a of the diode MBD 1. Furthermore, a junction exists between these two blocks 40 and 42 and forms a healthy signal output, which is connected to the signal processing circuit 28. On the health signal output, a monitor voltage Vmon is provided, which is a health signal indicative of the state of diode MBD 1.

Fig. 5 shows an example implementation of the first and second voltage processing blocks 40 and 42 of the voltage divider circuit 26 in parallel with a main circuit breaker diode MBD1, which is here also considered a first diode. It can be seen that the first voltage handling block 40 comprises a first voltage dividing means (here in the form of a first resistor R1) connected in parallel with a first reactive means (here in the form of a first capacitor C1), while the second voltage handling block 42 comprises a series circuit comprising a second voltage dividing means (here in the form of a second resistor R2) connected in series with a second unidirectional conducting element (here exemplified by a second diode D2). A second reactive component, here in the form of a second capacitor C2, is connected in parallel with the series circuit. It can also be seen that the second diode D2 has an opposite orientation compared to the first diode MBD 1. More particularly, the cathode of the second diode D2 is connected to the anode a of the first diode MBD1 and the anode is connected to the second resistor R2. Since the reactive components of both blocks are capacitors, it can be seen that they are of the same type.

Fig. 6 schematically illustrates an example implementation of the signal evaluation circuit 28. The signal evaluation circuit 28 comprises three inputs each receiving a monitoring signal Vmon, wherein a first input is connected to a first comparator 44, which first comparator 44 is in turn connected to a monitoring logic 56 via a first resistor 50, a second input is connected to a second comparator 46, which second comparator 46 is in turn connected to the monitoring logic 56 via a second resistor 52, and a third input is connected to a third comparator 48, which third comparator 48 is in turn connected to the monitoring logic 56 via a third resistor 54. The monitoring logic 56 has an output (here exemplified by a Light Emitting Diode (LED)) connected to a light emitter 60 via a fourth resistor 58. A comparator is provided to compare the voltage Vmon to a corresponding voltage threshold in order to indicate the state of diode MBD1, and monitoring logic 56 may encode the state of the diode for provision to the main control unit.

There are several ways in which the monitoring logic may be provided. The monitoring logic may be provided as a processor with associated memory that includes software code implementing its functionality. The monitoring logic may also be implemented as a special purpose circuit, such as an Application Specific Integrated Circuit (ASIC) or a Field Programmable Gate Array (FPGA).

It may also be mentioned here that the health investigation device 22 may be set at a high potential, while the main control unit may be set at a low potential, such as ground potential. Also, the LED may be replaced with a laser. In addition, electrical and electrical cables may be used in place of the light emitters and optical fibers.

Fig. 7 shows an example implementation of the power supply circuit 24, wherein the first voltage-dividing block 30 is implemented as a third resistor R3, the second voltage-dividing block is implemented as a zener diode Z2 in parallel with a third capacitor C3, wherein the anode of the zener diode Z2 is connected to the anode of the first diode MBD1, the third voltage-dividing block 36 is implemented as a fourth resistor R4, the fourth resistor R4 is connected in series with the parallel circuit consisting of the fifth variable resistor R5 and the zener diode Z5, the anode of the first zener diode Z5 is connected to the second junction, and the fourth voltage-dividing block is implemented as a zener diode Z3 in parallel with the fourth capacitor C4, wherein the cathode of the zener diode Z3 is connected to the anode of the first diode MBD 1. The selection block 35 may be implemented as two strings connected between a junction associated with the voltage divider block and the first input of the first DC/DC converter 34. In the example of fig. 7, the selection block 35 is implemented as a first string with a diode D3 connected between the first junction and the first input of the first DC/DC converter 34 and as a second string with a diode D4 and a second inverting DC/DC converter 62 connected between the second junction and the first input of the first DC/DC converter 34. In this case, the diode D4 of the second string is placed after the second inverter 62, i.e. the second inverter 62 is connected to the first input of the first DC/DC inverter 34 via the diode D4. In addition, both diodes D3 and D4 are connected with their cathodes to the first input of the first DC/DC converter 34. As an example, the second DC/DC converter 62 may be implemented as an inverting buck-boost converter, although other implementations are contemplated.

The function of the health survey device 22 will now be described in some more detail.

In case the transmission medium has a unidirectional power flow, a self-commutating semiconductor element or a unidirectional conducting element, such as a diode, may be used as a main breaker element in a circuit breaker connected in series with a power transmission medium, such as an electric line or an underground cable. This may be the case, for example, if the converter station uses current source converters.

In this case, a fault such as an earth fault or a voltage polarity reversal caused by the system during any type of fault may cause the current through the power transmission medium to reverse its current direction, which in turn causes the diode to be reverse biased and thereby shut down. This shutdown can then be achieved without the use of control signals. This may simplify and speed up fault handling in systems of the type described above. Diodes may also be advantageous for other reasons, such as for cost.

The diode may also need to monitor its health. However, this is not as simple.

The diode may have three states, where two of the states are associated with a normal diode and a third of the states is associated with a faulty diode. A healthy diode may be conducting current, in which case the diode is forward biased; or the diode may be blocking current, in which case the diode is reverse biased. The diode may also be faulty, in which case there is substantially zero voltage across the diode. Here, the reverse biased healthy diode is in a first reverse blocking state, the forward biased healthy diode is in a second forward conducting state, and the faulty diode is in a third faulty state.

However, when the diode is in the first state, the voltage across the diode has a first level, which is typically in the range of 3-5kV in absolute value. The first level also typically has a reversed polarity compared to the second state and also typically to the third state. When the diode is in the second state, the voltage across the diode has a second level having an absolute voltage that is much lower than the absolute value of the first voltage level. The second level is typically in the range of 0.8-1.35V. When the diode is in the third state, the voltage across the diode has a third level, typically about 0V or approximately 0V. It can thus be seen that the absolute value of the second voltage level is higher than the absolute value of the third voltage level, and the absolute value of the first voltage level is higher than the absolute value of the second voltage level. As an example, the absolute value of the first voltage level may be greater than ten times. Sometimes, the absolute value of the first voltage level may be more than one thousand times. In many cases, such as in the example above, the absolute value of the first voltage level is greater than two thousand times.

Now, when monitoring the health condition, it is often preferable to use the voltage across the diode in order to distinguish between the different states. It is also interesting to use as few different measurement voltages as possible in order to perform a health assessment. However, this is not easy to implement due to the very different voltage levels that occur for the different states. The challenge lies in the fact that: the diode voltage can be very low (diode-on state) and very high (diode-off state).

There is a need to monitor the health of diodes in circuit breakers in a simple and reliable manner. Therefore, it should be possible to identify and inform the main control unit about the diode operation mode together with information about the health of the diode.

Aspects of the present invention are provided to solve this problem.

This problem is basically solved using the voltage dividing circuit 26, wherein the first and second voltage dividing voltage processing blocks 40 and 42 can be regarded as constituting a third voltage divider that divides the voltage across the first diode MBD1 to be monitored to obtain the monitor voltage Vmon, both when forward biased and reverse biased. In this voltage divider circuit, the relationship between the first resistor R1 and the second resistor R2 is typically selected such that when the diode MBD1 is reverse biased, the voltage Vmon approaches the forward bias range of 0.8-1.35V. Therefore, the first voltage-dividing section R1 and the second voltage-dividing section R2 have a relationship therebetween that is set to divide the voltage of the reverse blocking state into levels that can be processed by the signal evaluation circuit 28. As an example, R1 may be in the range of one thousand times R2.

It can be seen in fig. 5 that for the reverse biased first diode MBD1, the second diode D2 will conduct and there will therefore be a partial voltage defined by R2/(R1+ R2) at the junction between the two voltage handling blocks 40 and 42, which will result in the voltage across the diode being divided by 1000 and thus the magnitude of Vmon will be a few volts in the case where R1 is 1000 × R2.

It can be seen that for the forward biased first diode MBD1, the second diode D2 will be reverse biased, and therefore the second resistor R2 is floating and will not participate in any voltage division. When the first unidirectional conductive element is forward biased (i.e., in a forward conductive state), the orientation of the second diode D2 thus removes the partial voltages of the first and second voltage-dividing components from the health signal as compared to the orientation of the first diode MBD 1. In this case, the first resistor R1 and the second capacitor C2 form an RC filter that filters the diode voltage. In this case, it can be seen that the monitor voltage Vmon is substantially the same as the forward biased diode voltage, i.e. about 0.8-1.35V. The monitor voltage Vmon representing the forward bias voltage will therefore be in approximately the same range as the monitor voltage Vmon representing the reverse bias voltage, but significantly lower than the divided first state voltage and thus can be readily distinguished. It may also be mentioned here that the voltages may have opposite polarities, one of which is positive and the other negative.

It can thus also be seen that the health signal Vmon is capable of representing all states of the first unidirectional conductive element.

In other words and for a more detailed analysis, this operation can be seen as follows:

a) reverse blocking state: when the diode MBD1 is reverse biased, a reverse bias blocking voltage (V) should be applied at the cathode terminal C of the diode D1 with respect to the anode a_RTypically 3 to 5 kV). When a reverse bias voltage is applied, the diode D2 in the voltage divider circuit is forward biased and the voltage division ratio at the healthy signal output is:

wherein, V_D2ONIs the voltage drop across the second diode D2 when the second diode D2 is forward biased. It can be seen that the voltage divider voltage has been shifted by the diode forward voltage V of D2_D2ONAnd this offset voltage can be taken into account for the signal evaluation circuit.

b) Forward bias state: during the time that diode MBD1 is forward biased, a forward bias voltage (VD — typically 0.8V to 1.35V) appears across the anode to the cathode. The voltage divider circuit shown in fig. 5 is referenced to local ground at the anode terminal. Thus, the voltage appearing at the cathode terminal C with respect to the anode terminal a of the diode MBD1 is the diode's negative forward voltage drop, and appears as a reverse bias voltage for the diode D2. With the diode D2 reverse biased, the resistor R2 becomes open and thus the voltage divider becomes an RC filter, with R1 in series with C2. Thus, the voltage division at the health signal output is:

wherein, V_DONIs the forward bias voltage of the first diode MBD 1.

It can also be seen that zero voltage will not be a problem for discrimination.

Thereby providing voltages in the same range and these voltages can then be provided to the signal evaluation circuit 28 for evaluation.

The signal evaluation circuit 28 receives a monitored voltage Vmon representing the voltage across the first diode MBD1, analyses the voltage Vmon in order to determine the state, and sends an indication of the state of the diode to a master control unit, which may be provided in the converter station control unit and is at ground potential. The indication may be sent using a light emitting element 60 and an optical fiber.

As an example, the monitored voltage may be compared to a first threshold value representing a first state in a first comparator 44, to a second threshold value representing a second state in a second comparator 46, and to a third threshold value representing a third state in a third comparator 48. The comparator crossing the threshold then sends a signal representing the corresponding state to the monitoring logic 56, which monitoring logic 56 in turn sends a signal representing the state to the main control unit via the optical fiber. Thus, it can be seen that the signal evaluation circuit 28 determines the state of the first unidirectional conductive element MDB1 by comparing the health indication signal Vmon to three different thresholds each associated with a corresponding state and indicating a state in which the corresponding threshold is crossed.

There are various ways in which the state can be obtained from the monitor voltage Vmon. Since the monitor voltage is used to indicate whether the diode is in a forward or reverse biased position, the monitor voltage may also be referred to as the diode position voltage.

a) As described above, different three comparators may be used which receive the diode position voltage Vmon through the voltage divider circuit 26. The main control unit may determine the diode operation mode from the monitoring logic using a simple protocol or may transmit the measured actual voltage as encoded data to the ground potential controller.

b) Alternatively, the energy in the monitoring capacitor C2 can be used to trigger a pulse train through the fiber. The width of the pulses should be the same, but the rate/frequency of arrival of the pulses is indicative of the operating mode/voltage of the diode. In this case, the signal evaluation circuit 28 encodes the voltage level of the health indicator signal Vmon into a pulse train having a pulse repetition rate corresponding to the voltage level of the health indicator signal Vmon so as to indicate the state. The pulse repetition rate of the first state may then be higher than the pulse repetition rate of the second state, which in turn may be higher than the pulse repetition rate of the third state, which may be zero. An example of such encoding can be seen in fig. 8, which shows a first state S1, a second state S2, and a third state S3. As can be seen, the diode is considered to be faulty if there is no pulse.

c) The third way is to sample the divided monitor diode position voltage Vmon using an analog-to-digital converter (ADC) and send it to ground potential through an optical fiber.

The signal evaluation circuit 28 requires external power in order to be able to carry out its function. Alternatively, the voltage divider circuit 26 may require such external power. Such power may be provided by a conventional power supply. However, in some cases there is no available network to which such a power supply can be connected, and in such cases the voltage across diode MBD1 may be used instead. This is the purpose of the power supply circuit 24.

Furthermore, as already stated above, in operation, the voltage across the diode may vary significantly. A power supply circuit 24 is provided to handle the variation and generate a stable supply voltage.

As can be seen in fig. 4, the power supply circuit 24 includes first, second, third and fourth voltage-dividing blocks 30, 32, 36 and 38. The voltage dividing block may be provided as a parallel circuit including a resistor connected in parallel with a capacitor. Optionally, there may also be a diode, such as a zener diode, in parallel with the resistor and/or the capacitor. There may also be a further resistor in series with the parallel circuit.

The first, second, third and fourth voltage divider blocks 30, 32, 36 and 38 divide the voltage across the diode MBD1 to generate power for powering the signal evaluation circuit 28. Due to the high voltage during the reverse bias state of diode MBD1 and the very low voltage in reverse polarity during the forward bias state of diode MBD1, two separate voltage dividers obtained by first and second voltage dividing blocks 30 and 32 and third and fourth voltage dividing blocks 36 and 38, respectively, are proposed. Here, the first voltage dividing block 30 and the second voltage dividing block 32 have a relationship with each other set to provide the voltage at the first node to a voltage of a reverse blocking state divided into levels that can be handled by the first DC/DC converter 34 when the first diode MBD1 is in the reverse blocking state. The third and fourth voltage dividers have a relationship to each other which in turn is arranged to provide a voltage corresponding to the forward bias voltage of the forward conduction state at a level which can be handled by the first DC/DC converter 34 when the first diode MBD1 is in the forward conduction state.

Then, the selection block 35 selects an input voltage to be applied between the input terminals of the first DC/DC converter 34 based on the voltage level and the polarity and reverses the polarity of the selected voltage as appropriate. Therefore, the selection block 35 selects whether to supply the voltage corresponding to the voltage at the second node of the forward conduction state or the divided voltage at the first node of the reverse blocking state to the first DC/DC converter 34. The selection block 35 may also reverse the polarity of one of the voltages.

When diode MBD1 is reverse biased, first and second voltage division blocks 30 and 32 are selected such that a voltage having a level that can be handled by inverter 34 is provided at a first junction between these blocks. For example, the voltage may be divided approximately 1000 times to a substantially lower level.

When the diode MBD1 is forward biased, the third and fourth voltage divider blocks 36 and 40 will be selected instead and divide the voltage across the diode MBD1 to a level that can be used by the inverter 34 as an input voltage. Since the diode voltage is already low, the division may be slightly off.

The inverter 34 then takes the input voltage selected by the selection block 33 from either a first junction between the first and second voltage divider blocks 30 and 32 or a second junction between the third and fourth voltage divider blocks 36 and 38. Thus, the first DC/DC converter 34 receives the input voltage from the voltage divider at the required level based on the diode operation mode. The DC/DC converter 34 then produces a smooth and stable output voltage for feeding to the signal evaluation circuit 28. The same first DC/DC converter 34 can be used for both modes of operation (on and off) of the diodes.

The power supply circuit 24 may be modified to some extent and the third and fourth voltage blocks omitted. This is schematically illustrated in fig. 9. The number of high voltage dividers can thus be reduced to one. This reduction can be achieved by optimizing the signal evaluation circuit in situations where power supply load, forward voltage drop, and reverse voltage isolation losses can be reduced. As previously described, when the first diode MBD1 is in the reverse blocking state, the first and second voltage division blocks 30, 32 have a relationship between each other of a voltage divided into levels that can be handled by the first DC/DC converter 34 set to provide a voltage at the first node as the reverse blocking state. However, in addition to this, when the first diode MBD1 is in the forward conducting state, the first and second voltage dividing blocks have a relationship with each other arranged to provide at the first node a voltage corresponding to the voltage in the forward conducting state at a level that can be handled by the first DC/DC converter. To achieve this, the first and second voltage dividing blocks 30 and 32 may be implemented in the same manner as the voltage processing block of the voltage dividing circuit. In this case, the selection block 35 may reverse the polarity of the applied voltage where appropriate.

In this case, one implementation of the selection block may be as two strings connected between the voltage dividing block and the junction between the first inputs of the first DC/DC converters. In the example of fig. 9, the selection block may be implemented as a first string with diodes connected between the first node and the first input of the first DC/DC converter and a second string with a second inverting DC/DC converter and diodes also connected between the first node and the second output of the first DC/DC converter. The two strings will be in parallel. Also in this case the diodes of the second string will be placed after the second converter, i.e. the second converter will be connected to the first input of the first DC/DC converter via the diodes. In addition, both diodes will be connected with their cathodes to the first input of the first DC/DC converter. Alternatively, there may be only one connection from the first node to any one input of the selection block. In this case, the first DC/DC converter may be an inverting DC/DC converter, if necessary.

The voltage dividing module of the power supply circuit and/or the voltage processing block of the voltage dividing circuit described above may be implemented using only passive components. In this respect it should also be mentioned that it is to be noted that the first DC/DC converter and the second DC/DC converter (if present) are self-operating DC/DC converters, wherein the operating power is taken from the converter input using the starting circuit. Alternatively, these converters may also be implemented using only passive components.

The invention has a number of advantages. Which provides a simple but effective and reliable way to monitor the health of a unidirectional conducting element in a circuit breaker, such as a diode in a diode valve. The diode voltage is fed to a signal evaluation circuit in which the diode operating mode and possibly also the health condition is determined. All these functions can be implemented with simple circuitry and with only one optical cable to the master control system at ground potential. Thus, the health of the diode is monitored in a simple and reliable manner. The main control unit, which is at ground potential, can be recognized and informed about the operating mode of the diode together with information about the health of the diode. This is done safely even if the monitoring voltage is both at a very low level (diode-on state) and at a high level (diode-off state). Additionally, it can be implemented without using any external power source. The health of the diodes can also be indicated with a simple protocol with the main control unit.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements. Accordingly, the invention is not to be restricted except in light of the attached claims.