Protection circuit of a three-phase electrical power supply system

Abstract

The invention relates to a protection circuit (2) for a medium-voltage power supply system (4), the power supply system having three phase lines (L1, L2, L3), the protection circuit comprising: - at least three first current transformers (10, 12, 14), the first current transformers (10, 12, 14) each having a primary side and a secondary side, the primary side of each first current transformer being coupled to a different phase line (L1, L2, L3), - at least three second current transformers (16, 18, 20), the second current transformers each having a primary side and a secondary side, the primary sides of the second current transformers being connected to the secondary sides of the first current transformers, - at least two thermal fuses (28), the thermal fuses (28) each being connected in parallel to the primary sides of the second current transformers.Figure to be published with the abbreviation: Single figure.

Description

Title of the invention: Protection circuit for a three-phase electrical power supply system Technical field of the invention

[0001] The present invention relates to a protection circuit for a medium-voltage power supply system using a trip actuator. Prior art

[0002] A protection circuit comprising three current transformers each directly coupled to a phase line and three thermal fuses connected between the output terminals of the current transformer is known from document EP 2 079 140.

[0003] However, this protection circuit includes current transformers coupled to the phase lines having a large volume.

[0004] To reduce costs, it is desirable to use small, low-power current transformers. These transformers cannot deliver such high power, which hinders their use with the known component chains of trigger actuators.

[0005] It is also known to have two or more electromagnets in the chain of components of the trigger actuator to trigger the interruption of the power supply system, which takes up space within these components.

[0006] Finally, the number of hot melt variants needed to cover the range of triggering characteristics desired by users is generally high. Presentation of the invention

[0007] A first objective of the present invention is to propose a compact protection circuit in particular by using low consumption feedthrough transformers coupled to the phases.

[0008] A second object of the present invention is to reduce the number of fuse variants to simplify the operation of the power supply system.

[0009] A third object of the present invention is to provide a protection circuit with low manufacturing cost and low electrical consumption.

[0010] A fourth object of the present invention is to provide a reliable and robust protection circuit. Summary of the invention

[0011] The present invention relates to a protection circuit for a medium-voltage power supply system, the power supply system having three phase lines, the protection circuit comprising: - at least three first current transformers, each first current transformer having a primary and a secondary side, the primary side of each first current transformer being coupled to a different phase line, - at least three second current transformers, each second current transformer having a primary and a secondary side, the primary sides of the second current transformers being connected to the secondary sides of the first current transformers, - at least two thermal fuses, each thermal fuse being connected in parallel to the primary sides of the second current transformers.

[0012] Advantageously, this protection circuit operates with small, low-cost fuses that are easy to change because they do not require intervention on the medium voltage side of the cell.

[0013] Advantageously, the second current transformers perform current matching and voltage matching between the secondaries of the first current transformers and the input of the trip actuator.

[0014] The features described in the following paragraphs may optionally be implemented. They may be implemented independently of each other or in combination with each other: - the terminals on the primary sides of the second current transformers are connected together at a connection point in a star configuration and in which the protection circuit includes at least one earth terminal connected to the connection point. Advantageously, the protection circuit detects overcurrent faults and earth faults. - the first current transformers included through-hole type transformers.

[0015] - The circuit includes a single trigger actuator configured for trigger a circuit breaker, and at least three current rectifiers, the current rectifiers each being connected in input to the secondary sides of the second current transformers, the current rectifiers being connected in output to said tripping actuator. - the second current transformers are configured to clip the current intensity at the output of the first current transformers. - the second current transformers have a saturation voltage between one and two times the operating voltage of the trigger actuator. - The thermal fuses are chosen from fuses having a nominal current chosen from 3 Amps, 5 Amps and 7.5 Amps. Advantageously, the protection circuit can operate over a wide voltage range with a reduced number of fuse references. Advantageously, the diameter of the electrical wires is smaller. Advantageously, there is less power loss in the circuit components. Advantageously, the protection circuit is less expensive. - The ratios of the number of windings between the primary side and the secondary side of the second current transformers are 50 / 5, 100 / 5 and 200 / 5. Advantageously, the second current transformers perform a clipping role on the currents entering the second transformers. Advantageously, the number of windings in the first transformers is reduced. Advantageously, the production cost of the first secondary transformers is reduced. Advantageously, the first current transformers do not have specific windings for the trip actuator. This results in greater compactness in a location where available space is limited, particularly due to the insulation between the medium-voltage and low-voltage sections. - The circuit includes resistors, each connected in parallel and at the input of each rectifier.

[0016] The invention also relates to an electrical circuit comprising: - a medium-voltage power supply system, the power supply system having three phase lines, - a circuit breaker configured to cut off the current on the phase lines, - a protection circuit as mentioned in the characteristics above. Brief description of the figures

[0017] [Fig-1] is a schematic view of an electrical circuit according to the invention. Detailed description of the invention

[0018] In this description, the terms "upstream" and "downstream" are to be interpreted according to the direction of the current in the protection circuit.

[0019] The invention relates to an electrical circuit 1 and a protection circuit 2. With reference to the single figure, the electrical circuit 1 comprises an electrical supply system 4, a protection circuit 2 coupled to the electrical supply system 4 and a circuit breaker 8 configured to cut off the current on the electrical supply system 4. Power supply system 4 is a medium voltage power supply system.

[0020] The term "medium voltage" refers to voltages above 1 kV and below 52 kV. The power supply system 4 is configured for to carry a three-phase alternating current. For this purpose, the electrical supply system 4 has three phase lines L1, L2, L3.

[0021] The circuit breaker 8 is connected to the three phase lines upstream of the protection circuit 2. In the embodiment illustrated in the figure and not limiting in any way, the circuit breaker 8 comprises three contacts each connected to a phase line.

[0022] The protection circuit 2 is connected downstream of the power supply system 4. The protection circuit 2 comprises three current transformers 10, 12, 14, referred to as first current transformers in the following description. Preferably, these current transformers are bushing transformers. These current transformers have a medium-voltage side, called the primary side, and a low-voltage side, called the secondary side. The primary side of each first current transformer is coupled to a phase line L1, L2, L3 of the power supply system.

[0023] The protection circuit 2 further includes three current transformers 16, 18, 20 hereinafter referred to as second current transformers, three earth terminals 24, 25, 27 and two thermal fuses 28.

[0024] The second current transformers 16, 18, 20 each have a primary and a secondary side. Each second current transformer 16, 18, 20 is connected to a different first current transformer. A terminal 22 on the secondary side of the first current transformers 10, 12, 14 is connected to the primary side of the second current transformers 16, 18, 20. The other terminals 26 on the secondary side of the first current transformers 10, 12, 14 are each connected to a ground terminal 24, 25, 27.

[0025] Two second current transformers 16, 18 are adapted to detect an overcurrent. They are hereinafter referred to as second overcurrent current transformers 16, 18. The other second current transformer 20 is adapted to detect an earth fault. It is hereinafter referred to as the second earth fault transformer 20.

[0026] A thermal fuse 28 is connected in parallel to the primary side of a second overcurrent transformer 16. The other thermal fuse 28 is connected in parallel to the primary side of the other second overcurrent transformer 18. The downstream terminals 30, 32 on the primary side of the second overcurrent transformers 16, 18 are connected to an upstream terminal 34 of the second earth fault current transformer 20 at a connection point 35 in a star configuration. The downstream terminal 36 on the primary side of the second earth fault current transformer 20 is electrically connected to each earth terminal 24, 25, 27.

[0027] The second current transformers 16, 18, 20 are configured to clip the output current of the first current transformers 10, 12, 14. In particular, the second current transformers 16, 18, 20 have a secondary saturation voltage between one and two times the operating voltage of the trip actuator. For example, the saturation voltage is between 4 and 8 volts.

[0028] Finally, the protection circuit 2 includes three current rectifiers 38, 40, 42 and a trip actuator 44. Each current rectifier is connected in input to the secondary side of the second current transformers 16, 18, 20 and in output to the trip actuator 44.

[0029] Preferably, the current rectifiers 38, 40, 42 are configured to clip the voltage and thus limit the power transmitted to them. Advantageously, this clipping ensures that the trip actuator will not be damaged in the event of a short circuit in the primary winding of the transformers.

[0030] Rectifiers 38, 40, 42 are, for example, made up of diode bridges. Preferably, the diode bridges comprise Schottky diodes.

[0031] The trip actuator 44 is suitable for tripping the circuit breaker 8. The trip actuator 44 is, for example, an electromagnet trip actuator. Advantageously, the protection circuit 8 comprises a single trip actuator. Thus, the protection circuit is smaller and less expensive.

[0032] Preferably, the protection circuit 2 comprises three resistors 50, 52, 54 each connected in parallel at the input of each rectifier 38, 40, 42. Advantageously, these resistors make it possible to limit overvoltages and to finely adjust the level of the tripping threshold.

[0033] Preferably, the protection circuit 2 further comprises a capacitor 56 connected in parallel with the release actuator.

[0034] Preferably, the protection circuit 2 includes an earth terminal 58 connected to the negative terminal of the trip actuator.

[0035] In normal operation, when a medium-voltage alternating current is carried by the phase lines L1, L2, L3, an alternating current is transmitted to the second current transformers 16, 18, 20 via the first current transformers 10, 12, 14. The primary current has an amplitude of approximately 5 Amperes. This current flows mainly through the thermal fuses 28. A low-amplitude residual current is transmitted to the secondary side of the second Current transformers 16, 18, 20. This current is rectified by current rectifiers 38, 40, 42. This residual current has a very low amplitude. This amplitude is well below the tripping threshold of the tripping actuator 44. Circuit breaker 8 is not tripped.

[0036] In the event of a short circuit on one of the phase lines L1, L2, L3, a high-amplitude current is transmitted to the primary side of at least one of the two secondary overcurrent transformers 16, 18 and to at least one of the two thermal fuses 28. This current has an intensity greater than the rated current of the thermal fuses 28, causing them to melt. The resistance of the thermal fuses 28 increases. The intensity of the current transmitted to the secondary side of the secondary overcurrent transformers 16, 18 also increases. This high-intensity current is rectified by the rectifiers 38, 42. It exceeds the tripping threshold of the trip actuator 44. The trip actuator 44 trips the circuit breaker 8.

[0037] In the event of a ground fault on one of the phase lines L1, L2, or L3, the currents arriving at the connection point 35 no longer have the same magnitude. The system is no longer balanced. A current flows on the line 45 connecting the connection point 35 to the ground output terminal 24. A current is transmitted to the secondary side of the second ground fault transformer. This current is rectified by the rectifier 40. This current is then transmitted to the trip actuator 44 and causes the circuit breaker 8 to trip.

Claims

Demands

1. Protection circuit (2) for a medium-voltage power supply system (4), the power supply system having three phase lines (L1, L2, L3), the protection circuit comprising: - at least three first current transformers (10, 12, 14), the first current transformers (10, 12, 14) each having a primary side and a secondary side, the primary side of each first current transformer being coupled to a different phase line (L1, L2, L3), - at least three second current transformers (16, 18, 20), the second current transformers each having a primary side and a secondary side, the primary sides of the second current transformers being connected to the secondary sides of the first current transformers, - at least two thermal fuses (28), the thermal fuses (28) each being connected in parallel to the primary sides of the second current transformers.

2. Protection circuit (2) according to claim 1, wherein the terminals of the primary sides of the second current transformers (16, 18, 20) are connected together at a connection point (35) in a star configuration and wherein the protection circuit includes at least one earth terminal (24, 25, 27) connected to the connection point (35).

3. Protection circuit (2) according to any one of claims 1 and 2, wherein the first current transformers (10, 12, 14) comprise through-hole type transformers.

4. A protection circuit (2) according to any one of claims 1 to 3, comprising a single trip actuator (44) configured to trip a circuit breaker (8), and at least three current rectifiers (38, 40, 42), the current rectifiers (38, 40, 42) each being connected in input to the secondary sides of the second current transformers (16, 18, 20), the current rectifiers being connected in output to said trip actuator (44).

5. Protection circuit (2) according to claim 4, wherein the second current transformers (16, 18, 20) are configured to clip the current intensity at the output of the first current transformers (10, 12, 14).

6. Protection circuit (2) according to any one of claims 4 and 5, wherein the second current transformers (16, 18, 20) have a saturation voltage between one and two times the operating voltage of the trip actuator.

7. Protection circuit (2) according to any one of claims 1 to 6, wherein the thermal fuses (28) are selected from fuses having a current rating selected from 3 Amperes, 5 Amperes and 7.5 Amperes.

8. Protection circuit (2) according to any one of claims 1 to 7, wherein the ratios of the number of windings between the primary side and the secondary side of the second current transformers are 50 / 5, 100 / 5 and 200 / 5.

9. Protection circuit (2) according to any one of claims 4 to 8, comprising resistors (50, 52, 54) each connected in parallel at the input of each rectifier (38, 40, 42).

10. Electrical circuit (1) comprising: - a medium-voltage power supply system (4), the power supply system having three phase lines (L1, L2, L3), - a circuit breaker (8) configured to interrupt the current on the phase lines, - a protection circuit (2) conforming to any one of claims 4 to 9.

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