Reverse polarity protection system and device in an HVDC electrical system

Abstract

In an HVDC electrical system (200), a protection system (100) includes a switch system (110) and an inductor (120) in series with a pole conductor (214). The protection system (100) also includes a reverse polarity protection device (1). The protection device (1) comprises a varistor branch (2) and a diode branch (3) in parallel to the varistor branch (2). The diode branch (3) can also include resistors (33), and the varistor and diode branches (2, 3) can be connected to each other at intermediate points, and included in the same insulator casing (4). In the event of a fault, the diode branch (3) prevents a pole conductor (214) of the HVDC system (200) from reversing polarity.

Description

[0001] Title: “Reverse polarity protection system and device in an HVDC electrical system”.

[0002] DESCRIPTION

[0003] Technical field

[0004] The present invention is developed in the technical field of the transmission of electrical energy at direct current high or very high voltage.

[0005] State of the art

[0006] High or very high voltage direct current electrical systems, known as HVDC, are becoming more widespread in some applications as a replacement for traditional alternating current lines.

[0007] HVDC systems require more complex electrical protections than the corresponding ones used for alternating current systems. One of the reasons for this is that while alternating current, even in the event of a fault, repeatedly passes through zero, this is not the case with direct current, and thus opening the circuit requires zeroing the current by dissipating the energy associated with it due to the inductive nature of the grid.

[0008] Recent protection systems for HVDC are installed at the start of the line and are illustrated in Figure 1. They comprise a switch, with a branch of IGBT-type transistors in parallel, which in turn have dischargers, also known as varistors, in parallel. In series with these components, an inductor is usually provided.

[0009] In the event of a fault, the current increases for a certain period of time before the protections intervene, as shown in Figure 7. Therefore, the switch is first opened, causing the current to flow into the IGBTs. The IGBTs are then also opened, causing a reduction in current and a consequent increase in voltage on the IGBTs themselves, caused by the inductance of the system. The voltage on the IGBTs reaches the clamping threshold of the varistors, which protect the IGBTs by limiting the voltage increase and letting the current flow through. In this way, the varistors dissipate electrical power and progressively reduce the current to zero.

[0010] In this context, the varistor clamping threshold is selected, in terms of voltage, so as to ensure that the current is zeroed within a predefined time.

[0011] Problem of the prior art

[0012] In the protection systems for HVDC described above, from the time the switch and the IGBTs are opened, and the current flows through the varistors, the voltage on the varistors reaches such a value that the HVDC line is temporarily subject to the phenomenon known as polarity reversal, for the entire portion of the line between the inductor and the fault point, and also, with maximum intensity, upstream of the inductor. Specifically, the voltage upstream of the inductor is shown in one of the voltage time shapes in Figure 8, and the voltage downstream of the inductor, on the line, is shown in the lowest of the voltage time shapes in Figure 9.

[0013] Therefore, assuming that, in normal operation, a line conductor is subject to a positive voltage, close to a certain rated value, during the extinguishing of the fault current this conductor is suddenly subject to a negative voltage, with a modulus comparable to the rated voltage and potentially higher, depending on the sizing of the line and the protections.

[0014] For various components of the line, this condition represents a source of stress, due to the previous accumulation of charges of opposite polarity over a prolonged period of time, with negative consequences known to those skilled in the art.

[0015] It is known that an additional set of varistors can be connected downstream of the inductor, between a pole conductor of the HVDC system and a point at earth potential, as shown in Figure 2. These varistors are usually enclosed in an isolator casing. This protects the system from positive or negative overvoltages that significantly exceed the rated voltage, but does not prevent polarity reversal.

[0016] Summary of the invention

[0017] The purpose of the present invention is to solve the problems mentioned with reference to the prior art, and in particular to prevent the occurrence of the polarity reversal phenomenon.

[0018] These and other purposes are achieved by a protection system for a high- voltage direct current HVDC electrical system and an HVDC electrical system, according to any one of the appended claims.

[0019] The invention provides that, where the prior art employed varistors downstream of the inductor, a branch of diodes is connected in parallel to the varistors. These diodes remain in a non-conducting state during normal operation of the HVDC system, and then enter a conducting state under the fault conditions commented above, keeping the line voltage temporarily and essentially at zero, preventing it from reversing.

[0020] Preferred embodiments also include resistors in series with the diodes, which help dissipate the energy of the fault current and regulate the system’s reaction time constants.

[0021] In preferred embodiments, the diode branch and the varistor branch can be enclosed in a single isolator casing. Therefore, the diode and varistor assembly can be supplied as a single device to prevent polarity reversal, with no additional footprint compared to the isolator with varistors alone. This device can also be conveniently installed in existing systems, replacing the varistor-only isolator, without significant structural changes or increases in bulk and volume.

[0022] Also described is a reverse polarity protection device for an HVDC high- voltage electrical system comprising a first terminal and a second terminal, and a first varistor branch comprising a plurality of varistors connected in series between the first terminal and the second terminal and sized for operation with a high or very high voltage applied between the first terminal and the second terminal. A diode branch comprises a plurality of diodes connected in series, according to a common polarity orientation, between the first terminal and the second terminal.

[0023] Further features and advantages of the invention will be recognisable to a person skilled in the art by the following detailed description of exemplary embodiments of the invention.

[0024] Brief description of the figures

[0025] For a better understanding of the following detailed description, some embodiments of the invention are illustrated in the accompanying drawings, wherein:

[0026] - Figures 1 and 2 schematically show HVDC electrical systems according to the prior art under fault conditions,

[0027] - Figure 3 schematically shows an HVDC electrical system comprising a protection system with a polarity reversal protection device, according to an embodiment of the invention, under fault conditions,

[0028] - Figure 4 schematically shows the reverse polarity protection device of the electrical system in Figure 3,

[0029] - Figure 5 shows a characteristic voltage-current graph of a varistor branch, in a continuous line, and a reverse polarity protection device, in a dashed line,

[0030] - Figure 6 schematically shows a switch system of the electrical system in Figure 3,

[0031] - Figure 7 shows a time shape graph of current in the event of a fault in the electrical system in Figure 3,

[0032] - Figure 8 shows a comparative graph of two time shapes of the voltage at a node between a switch system and an inductor, for the electrical systems in Figures 1 and 3, in the event of a fault, and

[0033] - Figure 9 shows a comparative graph of two time shapes of the voltage at a node downstream of an inductor, for the electrical systems in Figures 1 and 3, in the event of a fault.

[0034] DETAILED DESCRIPTION

[0035] A reverse polarity protection device for an HVDC high-voltage electrical system is collectively denoted by the number 1. The device 1 in use is part of a protection system for an HVDC high-voltage electrical system, denoted by the number 100. The number 200, on the other hand, is used to denote the HVDC high- voltage direct current electrical system.

[0036] First, some known features of the HVDC system 200 are introduced, in order to better understand those of the protection device 1 and the protection device 100.

[0037] The HVDC system 200 comprises a transmission line 210, e.g. an overhead or cable line. The transmission line 210 has a first end portion 211, a second end portion 212, and an intermediate portion 213 between the first and second end portions 211, 212.

[0038] Generally, the HVDC system 200 comprises at least one grid equivalent source 220 configured to power the transmission line 210. The grid equivalent source 220 is connected to the transmission line 210 at the first end portion 211. In the following, expressions such as “downstream” and “upstream” will be understood to mean that what is upstream is closer to the first end 211 and / or the grid equivalent source 220, and what is downstream is closer to the second end 212.

[0039] In some embodiments, the grid equivalent source 220 comprises a power supply device, e.g. an electronic power converter with at least one DC side. Alternatively, the grid equivalent source 220 comprises a connection node to one or more further transmission lines. In both cases, the grid equivalent source 220 can be modelled circuitously as a voltage generator.

[0040] More specifically, the transmission line 210 can be included in a radial power grid, in which case power is usually transmitted from the first end portion 211 to the second end portion 212. Alternatively, the transmission line 210 can be included in a meshed grid, and thus both end portions 211, 212 can be connected to grid equivalent sources 220 as connection nodes to other transmission lines.

[0041] The transmission line 210 comprises a positive pole portion and a negative pole portion. The transmission line 210 is configured for the application of a predetermined DC overall rated voltage between the positive pole portion and the negative pole portion. The overall rated voltage can be a high or extra-high voltage, i.e. not less than 35kV.

[0042] At least one of the positive pole portion and the negative pole portion comprises a pole conductor 214 extending from the first to the second end portions 211, 212.

[0043] In particular, the transmission line 210 can be either monopolar or bipolar. In the case of a monopolar line 210, either the positive pole portion or the negative pole portion does not comprise a pole conductor 214 extending across the entire transmission line 210, but coincides with an earth potential portion 215 of the HVDC system.

[0044] In the case of a bipolar line 210, the positive and negative pole portions may comprise positive and negative pole conductors 214, respectively. Each pole conductor 214 is configured for application of a rated voltage to earth 215 equal to the rated operating voltage of the HVDC system 200, between positive or negative pole and earth 215.

[0045] Figure 3 can be considered representative of a monopolar embodiment, in which the negative pole portion is identified by the ground, or of a bipolar embodiment, in which the negative pole portion is not represented.

[0046] The HVDC system 200 comprises a protection system 100, preferably arranged at the first end portion 211, in particular between the grid equivalent source 220 and the intermediate portion 213 of the transmission line 210. In the case of a bipolar line 210, the HVDC system 200 may comprise two protection systems 100 arranged at the first end portion 211, one for each pole conductor 214.

[0047] Optionally, the HVDC system comprises one or two additional specular protection systems 100 at the second end portion 212. In the following, only the one shown in Figure 3 and connected to the first end portion 211 will be described for simplicity.

[0048] The protection system 100 comprises a DC switch system 110, in use connected in series to the pole conductor 214 of either the positive or negative pole portion.

[0049] With reference to Figure 6, the switch system 110 generally comprises a switch 111 and a discharger device identified by a varistor branch 112, connected in parallel to the switch 111.

[0050] In its preferred embodiment, the switch I l l is generally an electromechanical switch, in particular an extra-fast switch. In addition, the switch system 110 further comprises a transistor, e.g. IGBT, branch 113 connected in parallel to the switch 111 and to the varistor branch 112. Additional transistors can be placed in series with the switch 111.

[0051] The transistor branch 113 can be regarded as a static-type switch. More specifically, varistor subgroups of the varistor branch 112, e.g. single varistors, are connected in parallel to transistor subgroups of the transistor branch 113, e.g. single transistors.

[0052] However, embodiments in which the switch system 110 is made in alternative known ways, for example by including, in addition to the varistor branch 212, only one of the switch 111 and the transistor branch 113, are not excluded.

[0053] The protection system 100 also comprises an inductor 120, connected in series with the switch system 110, preferably between the switch system 110 and the intermediate portion 213 of the transmission line 210. Thus, the inductor 120 in use is also connected in series to the same pole conductor 214 to which the switch system 110 is also connected.

[0054] In this description, the term inductor 120 is not used in general to designate any electrical component having a non-zero inductance parameter, but specifically a physical device, of a known type, equipped with a coil of conductive windings configured to generate a magnetic field in a central area when current flows through it. A ferromagnetic core may or may not be provided in the central area.

[0055] The protection system 100 of Figure 3 includes a reverse polarity protection device 1, with particular reference to Figure 4. The protection device 1 comprises a first terminal 11 configured for connection to the pole conductor 214 and a second terminal 12 configured for connection to an earth potential point 215 of the HVDC system 200. The first terminal 11 of the protection device 1 is connected in series with the inductor 120.

[0056] The protection device 1 is generally arranged between the inductor 120 and the intermediate portion 213 of the transmission line 210. In particular, the switch system 110 and the inductor 120 are arranged between the protection device 1 and the grid equivalent source 220. For example, the inductor 120 can be positioned between the switch system 110 and the protection device 1.

[0057] The protection device 1 comprises its own varistor branch 2. This will be referred to as the first varistor branch 2, to distinguish it from a second varistor branch, i.e. the already described varistor branch 112 of the switch system 110.

[0058] The first varistor branch 2 comprises a plurality of varistors 21 connected in series with each other. The first varistor branch 2 connects the first terminal 11 and the second terminal 12 of the protection device 1. Therefore, in use the first varistor branch 2 is subject to the rated voltage to earth of the HVDC system 200, i.e. the overall rated voltage or half of it, depending on the position of the protection device 1 and the monopolar or bipolar embodiment. In particular, the varistors 21 of the first varistor branch 2 are sized for operation with a high or very high voltage, or half such a voltage, applied between the first terminal 11 and the second terminal 12. Preferably, the varistors 21 are metal oxide varistors, e.g. zinc oxide.

[0059] It should be noted that every varistor has a known non-linear resistance, and in particular a high resistance when a relatively low voltage is applied to it, below a varistor clamping threshold, and a low resistance for relatively high voltages, above its own clamping threshold. The resistance values indicated are to be understood as differential resistances, i.e. a derivative of the voltage-current graph characterising the varistor. This is shown in Figure 5, where the continuous line represents a typical characteristic graph of a branch consisting of a series of varistors.

[0060] The varistors 21 of the first branch 2, being placed in series with each other, identify an overall clamping threshold in terms of voltage between the first terminal 11 and the second terminal 12, i.e. where the transmission line voltage 210 is applied. These varistors 21 are sized in such a way that the overall clamping threshold is greater than the rated voltage to earth of the HVDC system. Thus, despite the presence of the first varistor branch 2 between the pole conductor 214 and the ground 215, these are substantially isolated from each other at least up to the rated voltage to earth, in particular up to the overall clamping threshold.

[0061] According to a characteristic aspect of the invention, the protection device 1 comprises a diode branch 3 comprising a plurality of diodes 31 connected in series with each other. The diode branch 3 connects the first terminal 11 and the second terminal 12 of the protection device 1, and is therefore connected in parallel to the first varistor branch 2. The diodes 31 are configured for operation with a high or very high voltage, or half such a voltage, applied between the first terminal 11 and the second terminal 12. The voltage-current characteristic of the protection device 1 then becomes the dashed characteristic in Figure 5, which is determined for the positive part by the typical characteristic of the first varistor branch 2, and for the negative part by the typical characteristic of the diode branch 3.

[0062] As is well known, diodes are oriented electrical components. In fact, they only prevent significant currents from flowing in one current direction. The diodes 31 of the diode branch 3 are connected in series with each other according to a common polarity orientation, meaning that they all prevent current from flowing in the same direction. In particular, they prevent the passage of current in the direction from the first terminal 11 to the second terminal 12. In an embodiment, not illustrated, the diode branch 3 and the first varistor branch 2 can be essentially independent of each other. Instead, in the preferred embodiment, the diode branch 3 and the first varistor branch 2 are interconnected as described below.

[0063] The diode branch 3 has a plurality of sub-branches 32 connected in series with each other at intermediate nodes. Each sub-branch 32 comprises at least one diode 3. Similarly, the first varistor branch 2 has a plurality of sub-branches 22 connected in series with each other at intermediate nodes. Each sub-branch 22 comprises at least one varistor 21. Each sub-branch 22 of the first varistor branch 22 is connected in parallel to a respective sub-branch 32 of the diode branch 3. In other words, the intermediate nodes are in common between the diode branch 3 and the first varistor branch 2.

[0064] Preferably, the protection device 1 comprises an isolator casing 4, which is configured for high voltage isolation in air, as such already known. The isolator casing 4 has an inner compartment, in which the first varistor branch 2 is enclosed, while the first terminal 11 and the second terminal 12 protrude from the isolator casing 4. In the preferred embodiment, the diode branch 3 is enclosed in the same isolator casing 4.

[0065] Preferably, the diode branch 4 comprises at least one resistor 33 connected in series to the diodes 31, plus preferably a plurality of resistors 33 connected in series to and alternating with the diodes 31. In particular, in the preferred embodiment, each sub-branch 32 of the diode branch 3 comprises at least one resistor 33.

[0066] The number of diodes 31 and resistors 33 of each sub-branch of the diode branch 3, and the number of varistors 21 of each sub-branch of the first varistor branch 2 may vary depending on the sizing of the components, since, for reasons of uniform voltage distribution, it is preferable for all sub-branches 32 of the diode branch 3 to be equal to each other, and similarly all sub-branches 22 of the first varistor branch 2 to be equal to each other.

[0067] In the following, the use of the protection device 1 and protection system 100 in the HVDC system, under operating conditions, and in particular when an electrical fault occurs, will be described with reference to graphs 7 to 9.

[0068] Prior to the fault, a voltage substantially equal to the overall rating is applied to the transmission line 210, between the positive pole portion and the negative pole portion, and so a voltage substantially equal to the rating to earth is applied to each pole conductor 214. The switch 111 and transistor branch 113 are in the conduction state, i.e. closed. Furthermore, a current in the transmission line is essentially constant. Therefore, there is no significant voltage drop on the switch system 110 and inductor 120, and the rated voltage to earth is also applied on the protection device 1.

[0069] When an electrical fault occurs along the transmission line 210, downstream of the protection system 100, e.g. in the intermediate portion 213 of the line, at the point of fault there is no isolation between a pole conductor 214 and the ground 215, i.e. there is contact between the two parts, possibly through a contact resistance.

[0070] The current does not rise instantaneously due to the presence of the inductor 120. In contrast, the inductor 120 and the portion of the pole conductor 214 upstream of the fault are together subjected to a voltage essentially equal to the rated voltage to earth.

[0071] The current thus increases linearly based on the sum of the inductances of the inductor and the portion of the pole conductor 214 upstream of the fault. The current increase is then interrupted when the switch 111 and the transistor branch 113 are opened. This therefore occurs at the moment of maximum current, as can be seen in Figure 7.

[0072] Again, the current does not decrease instantaneously due to the presence of the inductor 120. Instead, the current, having to continue circulating despite the opening of the switch 111 and the transistor branch 113, flows through the second varistor branch 112, bringing its voltage to its own clamping threshold.

[0073] It should be noted that, for fault current extinction, the overall clamping threshold of the second varistor branch 112 is greater than the rated voltage to earth of the HVDC system. For the purpose of rapid extinction, this clamping threshold can also be more than twice the rated voltage to earth. For this purpose, the second varistor branch 112 can be made in a similar way as described for the first varistor branch 2, except for different sizing.

[0074] Furthermore, it should be noted that under the described fault conditions a voltage substantially equal to the clamping threshold of the second varistor branch 112 is applied to the transistor branch 113. Therefore, the transistors 113 must be sized so that an overall blocking voltage of the transistor branch 113 is higher than the overall clamping threshold of the second varistor branch 112.

[0075] With the intervention of the second varistor branch 112, the node between the switch system 110 and the inductor 120 is at a reverse voltage with respect to normal operation, e.g. a negative voltage in the embodiment of the figures. In Figure 9, the voltage reversal upstream of the inductor 120 can be appreciated.

[0076] Should this reverse voltage be distributed across the inductor 120 and the portion of pole conductor 214 upstream of the fault, the voltage downstream of the inductor 120 would also be reversed. However, this is prevented by the diode branch 3. Thus, the node downstream of the inductor 120 is at essentially zero potential, i.e. the earth potential, except for voltage drops in resistors 33 of the diode branch 3, as is true for the entire portion of pole conductor 214 upstream of the fault. Thus, the current in the inductor 120 drops rapidly (Figure 7), without leading to polarity reversals on the portion of the pole conductor 214 upstream of the fault (Figure 9, curve remaining higher between milliseconds 5 and 7). The inductive energy associated with the current is dissipated in the second discharger branch 112. As shown in the comparative graph in Figure 8, the voltage upstream of the inductor 120 reaches steady state more quickly than in the case of the absence of the protection device 1.

[0077] In contrast, the portion of pole conductor 214 upstream of the fault lies between the diode branch 3 and the fault, i.e. two points at essentially zero potential. Therefore, its current decreases according to a resistive-inductive transient, due to the intrinsic inductance of the line 210 and the resistances of the line 210, fault and diode branch 3. The resistors 33 of the diode branch 3 can be sized to achieve extinguishing of these currents at a predetermined time. In some cases, resistors 33 may not be required, as intrinsic resistances of other system components, e.g. of the first discharger branch 2, are sufficient.

[0078] With the extinguishing of currents upstream and downstream of the protection device 100, the entire pole conductor 214 is at zero potential, while a voltage equal to the rated voltage to earth, below the clamping threshold of the second varistor branch 112, is applied to the switch system 110.

[0079] Obviously a person skilled in the art will be able to make numerous equivalent modifications to the variants set forth above, without thereby abandoning the scope of protection as defined by the appended claims.

Claims

CLAIMS1. Protection system (100) for an HVDC high voltage direct current electrical system (200), comprising:- a direct current switch system (110),- an inductor (120) connected in series with the switch system (110), wherein the switch system (110) and the inductor (120) are configured for connection in series to a pole conductor (214) of a positive pole portion or a negative pole portion of the HVDC system (200), characterised in that it comprises:- a reverse polarity protection device (1) comprising:- a first terminal (11) configured for connection to said pole conductor (214) and a second terminal (12) configured for connection to an earth potential point (215) of the HVDC system (200), wherein the first terminal (11) of the protection device (1) is connected in series with the inductor (120),- a first varistor branch (2), comprising a plurality of varistors (21) connected in series between the first and second terminals (11, 12) and sized for operation with a predetermined voltage applied between the first and second terminals (11, 12), the predetermined voltage being equal to an HVDC system operating voltage (200), and- a diode branch (3), comprising a plurality of diodes (31) connected in series, according to a common polarity orientation, between the first and second terminals (11, 12).

2. Protection system (100) according to claim 1, wherein the inductor (120) ispositioned between the switch system (110) and the protection device (1).

3. Protection system (1) according to claim 1 or 2, wherein:- the diode branch (3) has a plurality of sub-branches (32) connected in series, each sub-branch (32) comprising a diode (31),- the first varistor branch (2) has a plurality of sub-branches (22) connected in series, each sub-branch (22) comprising a varistor (21), and- each sub-branch (22) of the first varistor branch (2) is connected in parallel to a respective sub-branch (32) of the diode branch (3).

4. Protection system (100) according to any one of claims 1 to 3, wherein the diode branch (3) comprises at least one resistor (33) connected in series to the diodes (31), preferably a plurality of resistors (33) connected in series to the diodes (31) and alternating with the diodes (31).

5. Protection system (100) according to any one of claims 1 to 4, wherein the protection device (1) comprising an isolator casing (4), the first varistor branch (2) and the diode branch (3) being enclosed in the isolator casing (4).

6. Protection system (100) according to any one of claims 1 to 5, wherein the switch system (110) comprises:- a switch (111), and- a second varistor branch (112) connected in parallel to the switch (111).

7. Protection system (100) according to claim 6, wherein the switch (111) is anelectromechanical switch and the switch system (110) further comprises a transistor branch (113) connected in parallel to the switch (111) and the second varistor branch (H2).

8. High voltage direct current electrical system HVDC (200), comprising:- a transmission line (210) comprising a positive pole portion, a negative pole portion and at least one earth potential point (215), wherein at least one of the positive pole portion and the negative pole portion comprises a respective pole conductor (214),- a protection system (100) according to any one of claims 1 to 7, connected to said pole conductor (214) and to said earth potential point (215).

9. Electrical system according to claim 8, wherein the transmission line (210) has a first and second end portion (211, 212), and an intermediate portion (213) between the first and second end portions (211, 212), the protection system (100) being arranged at the first end portion (211).

10. Electrical system according to claim 9, comprising an grid equivalent source (220) for the transmission line (210) connected to the first end portion (211), the protection device (1) being arranged between the grid equivalent source (220) and the intermediate portion (213) of the transmission line (210), wherein preferably the grid equivalent source (220) is, for example, an electronic power converter, or a connection node to one or more further transmission lines.

Images