An AC cross-current suppression circuit and device for a station DC system

Abstract

The present invention provides an AC cross-current suppression circuit and device for a station DC system, wherein the suppression circuit includes two AC resistors, a resistor R1 and a resistor R2; the resistor R1 is connected between the positive DC bus and the ground, and the resistor R2 is connected between the negative DC bus and the ground; the first AC resistor is connected between the positive DC bus and the load, and the second AC resistor is connected between the negative DC bus and the load; the first AC resistor and the second AC resistor are the same, including a transformer T and a resistor R4; the two ends of the primary winding of the transformer T are respectively connected to the load and the positive DC bus or the two ends of the primary winding of the transformer T are respectively connected to the load and the negative DC bus; the resistor R4 is connected to the two ends of the secondary winding of the transformer T. The use of AC resistors in each branch of the present invention effectively suppresses the AC voltage that crosses into the DC system, and the branches will not affect each other. No isolated DC / DC converter is required, which saves cost and space.

Description

Technical Field

[0001] The present invention relates to the technical field of AC crosstalk suppression of a DC system in a station, and in particular to an AC crosstalk suppression circuit and device for a DC system for a station. Background Art

[0002] The DC system of the substation mainly supplies power to the secondary equipment. If the DC system fails, it will affect the normal operation of the substation equipment. The DC system is an ungrounded system. In order to effectively monitor the insulation of the DC system, balanced bridge resistors and unbalanced bridge resistors are usually connected. AC crossover is a special type of DC system grounding fault. On the one hand, the AC power supply is equivalent to the DC system being grounded through the AC power supply. On the other hand, the AC voltage is connected in series with the DC voltage to form a biased AC. Both situations will cause the switch to malfunction.

[0003] Most of the existing measures monitor AC cross-current inrush and issue an alarm, but they cannot suppress AC cross-current inrush. One method is to clamp AC cross-current through a diode, forcing the feeder branch circuit breaker that crosses into AC to trip. This method only clamps half of the cycle of AC cross-current inrush and cannot eliminate the impact of the other half-wave on the system. In addition, when the AC cross-current current is small, the feeder branch circuit breaker cannot be forced to trip, and the function of isolating the faulty branch cannot be achieved. A single AC cross-current inrush does not affect system operation but will force the circuit breaker to trip. Another method is to connect an isolated DC / DC converter in series with the feeder branch to isolate the impact of the fault on the branch, but it cannot isolate the AC power of the DC, and the connection of a converter in series with each branch directly leads to problems such as reduced power supply reliability, increased economic costs, and large installation space. Summary of the invention

[0004] The purpose of the present invention is to provide an AC crosstalk suppression circuit and device for a station DC system, which can solve the problems in the prior art that AC crosstalk cannot be effectively suppressed, and the cost of using an isolated DC / DC converter to suppress AC crosstalk is high and occupies a large space.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] In the first aspect, the present invention provides an AC crosstalk suppression circuit for a station DC system, comprising two AC resistors, a resistor R1 and a resistor R2; the resistor R1 is connected between the positive DC bus and the ground, and the resistor R2 is connected between the negative DC bus and the ground; the first AC resistor is connected between the positive DC bus and the load, and the second AC resistor is connected between the negative DC bus and the load; the first AC resistor and the second AC resistor are the same, comprising a transformer T and a resistor R4; in the first AC resistor, the two ends of the primary winding of the transformer T are respectively connected to the load and the positive DC bus; in the second AC resistor, the two ends of the primary winding of the transformer T are respectively connected to the load and the negative DC bus; the resistor R4 is connected to the two ends of the secondary winding of the transformer T.

[0007] Furthermore, several voltage acquisition circuits are used to respectively acquire the DC bus positive voltage to ground, the DC bus negative voltage to ground, the voltage across the resistor R1 and the voltage across the resistor R2 to further determine the specific location of the leakage current.

[0008] Furthermore, the voltage acquisition circuit includes a voltage divider circuit, a low-pass filter circuit, an isolation circuit, a voltage amplifier circuit and an output filter circuit. The two input ends of the voltage divider circuit are respectively connected to the two ends of the voltage to be collected, the output end of the voltage divider circuit is connected to the input end of the low-pass filter circuit, the output end of the low-pass filter circuit is connected to the input end of the isolation circuit, the output end of the isolation circuit is connected to the input end of the voltage amplifier circuit, and the voltage sampling signal outputted from the output end of the voltage amplifier circuit is collected after being filtered by the output filter circuit.

[0009] Furthermore, the voltage divider circuit includes resistors R7, R8 and R6; one end of the resistor R7 serves as an input end of the voltage divider circuit, the other end of the resistor R7 is connected to one end of the resistor R8, the other end of the resistor R8 is connected to one end of the resistor R6 and serves as an output end of the voltage divider circuit; the other end of the resistor R6 serves as another input end of the voltage divider circuit.

[0010] Furthermore, the low-pass filter circuit includes a resistor R9 and a capacitor C2, one end of the resistor R9 serves as an input end of the low-pass filter circuit, the other end of the resistor R9 is connected to the input end of the isolation circuit and one end of the capacitor C2, and the other end of the capacitor C2 is connected to the other end of the resistor R6.

[0011] Furthermore, the isolation circuit includes an isolation optocoupler U1, a capacitor C1, a capacitor C3, a capacitor C4, a capacitor C5 and an inductor L1, the positive input and negative input of the isolation optocoupler U1 serve as input ends of the isolation circuit, the positive input of the isolation optocoupler U1 is connected to the other end of the resistor R9, and the negative input of the isolation optocoupler U1 is connected to the other end of the resistor R6; the capacitor C1 is connected between the VDD1 pin and the negative input of the isolation optocoupler U1; the inductor L1 is connected between the voltage VCC+5V and the VDD2 pin of the isolation optocoupler U1; the capacitor C4 is connected between the VDD2 pin of the isolation optocoupler U1 and ground; the positive output and negative output of the isolation optocoupler U1 serve as output ends of the isolation circuit; the capacitor C3 is connected between the positive output of the isolation optocoupler U1 and ground, and the capacitor C5 is connected between the negative output of the isolation optocoupler U1 and ground.

[0012] Furthermore, the voltage amplifier circuit includes a resistor R3, a resistor R5, a resistor R10, a resistor R11, a resistor R12, a resistor R13, a resistor R14, a capacitor C6, a capacitor C7, a capacitor C8, a capacitor C9, an inductor L2 and an operational amplifier U3A; one end of the resistor R10 and one end of the resistor R11 serve as input ends of the voltage amplifier circuit, one end of the resistor R10 is connected to the positive output of the isolation optocoupler U1, and one end of the resistor R11 is connected to the negative output of the isolation optocoupler U1; the other end of the resistor R10 is connected to the in-phase input end of the operational amplifier U3A, and the other end of the resistor R11 is connected to the inverting input end of the operational amplifier U3A. phase input terminal; resistor R12 is connected between the non-inverting input terminal of the operational amplifier U3A and ground; the reference voltage is input to the non-inverting input terminal of the operational amplifier U3A through resistor R13; capacitor C9, resistor R3 and resistor R5 are respectively connected between the inverting input terminal and the output terminal of the operational amplifier U3A; the power supply voltage VCC3.3 is input to the positive voltage of the operational amplifier U3A through the inductor L2; capacitor C6 and capacitor C7 are connected between the positive voltage of the operational amplifier U3A and ground; capacitor C6 is a polar capacitor, and the negative end is connected to the ground; the output terminal of the operational amplifier U3A serves as the output terminal of the voltage amplifier circuit.

[0013] Furthermore, the output filter circuit includes a resistor R14 and a capacitor C8, one end of the resistor R14 is connected to the output end of the voltage amplifier circuit, the other end of the resistor R14 is connected to one end of the capacitor C8 and serves as a voltage sampling end; the other end of the capacitor C8 is grounded.

[0014] Furthermore, the basis for determining the specific location of the leakage current is:

[0015] If U+>0 or U->0, it indicates that there is an AC current fault in the DC bus;

[0016] If U R1n+>0, indicating that the positive electrode of the nth branch is connected to the alternating current;

[0017] If U R2n +>0, indicating that the negative pole of the nth branch is connected to the alternating current;

[0018] Among them, U+ is the AC voltage of the DC bus positive to the ground, U- is the AC voltage of the DC bus negative to the ground, and U R1n + is the voltage across the positive AC resistance of the nth branch, U R2n + is the voltage across the negative AC resistor of the nth branch.

[0019] In a second aspect, the present invention provides an AC current suppression device for a station DC system, comprising a housing, a printed circuit board and the above-mentioned AC current suppression circuit, wherein the AC current suppression circuit is welded on the printed circuit board and then sealed in the housing.

[0020] The AC voltage leakage suppression circuit and device of the station DC system of the present invention uses AC resistors in each branch, which can effectively suppress the AC voltage leakage into the DC system, and the branches will not affect each other. No isolated DC / DC converter is required, which saves cost and space. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0022] Figure 1 A circuit diagram of an AC cross-current suppression circuit for a station DC system of the present invention;

[0023] Figure 2 It is a circuit diagram of the voltage acquisition circuit of the present invention. DETAILED DESCRIPTION

[0024] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0025] The following describes the embodiments of the present disclosure through specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all of the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present disclosure.

[0026] Embodiment 1

[0027] The present invention provides an AC cross-current suppression circuit for a station DC system, such as Figure 1 As shown, it includes two AC resistors, resistor R1 and resistor R2. Resistor R1 is connected between the positive +KM of the DC bus and the ground, and resistor R2 is connected between the negative -KM of the DC bus and the ground. The first AC resistor is connected between the positive +KM of the DC bus and the DC load, and the second AC resistor is connected between the negative -KM of the DC bus and the DC load. The first AC resistor and the second AC resistor are the same, including a transformer T and a resistor R4. In the first AC resistor, the two ends of the primary winding of the transformer T are respectively connected to the load and the positive +KM of the DC bus; in the second AC resistor, the two ends of the primary winding of the transformer T are respectively connected to the load and the negative -KM of the DC bus. Resistor R4 is connected to the two ends of the secondary winding of the transformer T.

[0028] The main function of transformer T is to reduce the AC current of the loop and reduce the DC attenuation current. Transformer T is a small-capacity anti-saturation transformer with a transformation ratio of 1:1. The primary side of transformer T is connected to the feeder, and the secondary side is connected to the high-resistance resistor R4. Transformer T has the ability to only pass changing current and isolate DC current, so the high resistance R4 on the secondary side only works on AC and changing DC. When AC current is injected into the feeder, the AC current is limited due to the presence of high resistance R4. DC current can flow normally through the primary winding of transformer T and is not affected by the high resistance R4. The above-mentioned AC cross-current suppression circuit is connected to the DC bus as a branch, and there are several such branches on the DC bus. When there is an exponentially decaying DC component in the current of one of the branches, due to the presence of high resistance R4, the branch resistance increases and the DC transient current in the branch decreases.

[0029] Preferably, a voltage acquisition circuit is used to respectively acquire the DC bus positive +KM voltage to ground, the DC bus negative -KM voltage to ground, the voltage across resistor R1 and the voltage across resistor R2 to further determine the specific location of the leakage current.

[0030] Furthermore, the voltage acquisition circuit is as follows Figure 2 As shown, it includes a voltage divider circuit, a low-pass filter circuit, an isolation circuit, a voltage amplifier circuit and an output filter circuit. The two input ends of the voltage divider circuit are respectively connected to the two ends of the voltage to be collected, and the two ends of the voltage to be collected are at least one of the DC bus positive +KM and ground, the DC bus negative -KM and ground, the two ends of the resistor R1, and the two ends of the resistor R2. The output end of the voltage divider circuit is connected to the input end of the low-pass filter circuit, the output end of the low-pass filter circuit is connected to the input end of the isolation circuit, the output end of the isolation circuit is connected to the input end of the voltage amplifier circuit, and the voltage sampling signal output by the output end of the voltage amplifier circuit is collected after being filtered by the output filter circuit.

[0031] Furthermore, the voltage divider circuit includes a resistor R7, a resistor R8 and a resistor R6, one end of the resistor R7 serves as an input end of the voltage divider circuit, the other end of the resistor R7 is connected to one end of the resistor R8, the other end of the resistor R8 is connected to one end of the resistor R6 and serves as an output end of the voltage divider circuit. The other end of the resistor R6 serves as another input end of the voltage divider circuit.

[0032] Furthermore, the low-pass filter circuit includes a resistor R9 and a capacitor C2, one end of the resistor R9 serves as an input end of the low-pass filter circuit, the other end of the resistor R9 is connected to the input end of the isolation circuit and one end of the capacitor C2, and the other end of the capacitor C2 is connected to the other end of the resistor R6.

[0033] Furthermore, the isolation circuit includes an isolation optocoupler U1, a capacitor C1, a capacitor C3, a capacitor C4, a capacitor C5 and an inductor L1, wherein the positive input and the negative input of the isolation optocoupler U1 serve as the input terminals of the isolation circuit, the positive input of the isolation optocoupler U1 is connected to the other end of the resistor R9, and the negative input of the isolation optocoupler U1 is connected to the other end of the resistor R6. Capacitor C1 is connected between the VDD1 pin and the negative input of the isolation optocoupler U1. Inductor L1 is connected between the voltage VCC+5V and the VDD2 pin of the isolation optocoupler U1. Capacitor C4 is connected between the VDD2 pin of the isolation optocoupler U1 and the ground. The positive output and the negative output of the isolation optocoupler U1 serve as the output terminals of the isolation circuit. Capacitor C3 is connected between the positive output of the isolation optocoupler U1 and the ground, and capacitor C5 is connected between the negative output of the isolation optocoupler U1 and the ground.

[0034] Further, the voltage amplifier circuit includes resistor R3, resistor R5, resistor R10, resistor R11, resistor R12, resistor R13, resistor R14, capacitor C6, capacitor C7, capacitor C8, capacitor C9, inductor L2 and operational amplifier U3A, one end of resistor R10 and one end of resistor R11 are used as input ends of the voltage amplifier circuit, one end of resistor R10 is connected to the positive output of the isolation optocoupler U1, and one end of resistor R11 is connected to the negative output of the isolation optocoupler U1. The other end of resistor R10 is connected to the non-inverting input end of the operational amplifier U3A, and the other end of resistor R11 is connected to the inverting input end of the operational amplifier U3A. Resistor R12 is connected between the non-inverting input end of the operational amplifier U3A and ground. The reference voltage is input to the non-inverting input end of the operational amplifier U3A through resistor R13. Capacitor C9, resistor R3 and resistor R5 are respectively connected between the inverting input end and the output end of the operational amplifier U3A. The supply voltage VCC3.3 is input to the positive voltage of the operational amplifier U3A through the inductor L2. Capacitors C6 and C7 are connected between the positive voltage of the operational amplifier U3A and the ground. Capacitor C6 is a polarized capacitor, and one end of the negative electrode is connected to the ground. The output end of the operational amplifier U3A serves as the output end of the voltage amplifier circuit.

[0035] Furthermore, the output filter circuit includes a resistor R14 and a capacitor C8, one end of the resistor R14 is connected to the output end of the operational amplifier U3A, the other end of the resistor R14 is connected to one end of the capacitor C8 and serves as a voltage sampling end, and the other end of the capacitor C8 is grounded.

[0036] Figure 2 The working principle is as follows: V1+ and V1-GND are the two ends of the voltage to be collected. The collected voltage is divided by resistors R7, R8 and R6, and the voltage across resistor R6 is used as the input voltage of the isolation optocoupler U1. Resistor R9 and capacitor C2 form a low-pass filter circuit to filter out high-frequency components. After the voltage is divided, the weak voltage is isolated and amplified by the isolation optocoupler U1, capacitors C3 and C5 are used as output filters of the isolation optocoupler U1, and inductor L1 is the isolation optocoupler freewheeling.

[0037] Resistors R3, R5 and C9 serve as the negative feedback network of operational amplifier U3A, and together with other peripheral circuits of operational amplifier U3A, they form a voltage amplifier circuit. The power supply voltage VCC3.3 supplies power to operational amplifier U3A through capacitors C6, C7 and inductor L2. After voltage amplification, it is filtered by resistor R14 and capacitor C8 and then AD sampled as a basis for determining the specific location of AC crosstalk.

[0038] Furthermore, the basis for determining the specific location of AC cross-current is:

[0039] If U+>0 or U->0, it indicates that there is an AC current fault in the DC bus;

[0040] If U R1n +>0, indicating that the positive electrode of the nth branch is connected to the alternating current;

[0041] If U R2n +>0, indicating that alternating current is entering the negative pole of the nth branch.

[0042] Where U+ is the positive AC voltage of the DC bus to the ground, U- is the negative AC voltage of the DC bus to the ground, and U R1n + is the voltage across the positive AC resistor (i.e. R1) of the nth branch, U R2n + is the voltage across the negative AC resistor (i.e. R2) of the nth branch.

[0043] The present invention also provides an AC current suppression device for a station DC system, comprising a housing, a printed circuit board and the above-mentioned AC current suppression circuit. After the AC current suppression circuit is welded on the printed circuit board, it is sealed in the housing.

[0044] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, an electrical connection, or communication with each other; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] The above is only for explaining the implementation mode of the present invention and is not intended to limit the present invention. For those skilled in the art, any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention without creative work should be included in the protection scope of the present invention.

Claims

1. An AC cross-current suppression circuit for a station DC system. It is characterized in that The invention comprises two AC resistors, a resistor R1 and a resistor R2; the resistor R1 is connected between the positive of the DC bus and the ground, and the resistor R2 is connected between the negative of the DC bus and the ground; the first AC resistor is connected between the positive of the DC bus and the load, and the second AC resistor is connected between the negative of the DC bus and the load; the first AC resistor and the second AC resistor are the same, and comprise a transformer T and a resistor R4; in the first AC resistor, the two ends of the primary winding of the transformer T are respectively connected to the load and the positive of the DC bus; in the second AC resistor, the two ends of the primary winding of the transformer T are respectively connected to the load and the negative of the DC bus; the resistor R4 is connected to the two ends of the secondary winding of the transformer T; a plurality of voltage sampling The collecting circuit respectively collects the positive voltage of the DC bus to the ground, the negative voltage of the DC bus to the ground, the voltage across the resistor R1 and the voltage across the resistor R2 to further determine the specific location of the current leakage; the voltage collection circuit includes a voltage divider circuit, a low-pass filter circuit, an isolation circuit, a voltage amplifier circuit and an output filter circuit, the two input ends of the voltage divider circuit are respectively connected to the two ends of the voltage to be collected, the output end of the voltage divider circuit is connected to the input end of the low-pass filter circuit, the output end of the low-pass filter circuit is connected to the input end of the isolation circuit, the output end of the isolation circuit is connected to the input end of the voltage amplifier circuit, and the voltage sampling signal outputted from the output end of the voltage amplifier circuit is collected after being filtered by the output filter circuit.

2. The AC cross-current suppression circuit of the station DC system according to claim 1, It is characterized in that The voltage divider circuit includes resistors R7, R8 and R6; one end of the resistor R7 serves as an input end of the voltage divider circuit, the other end of the resistor R7 is connected to one end of the resistor R8, the other end of the resistor R8 is connected to one end of the resistor R6 and serves as an output end of the voltage divider circuit; the other end of the resistor R6 serves as another input end of the voltage divider circuit.

3. The AC cross-current suppression circuit of the station DC system according to claim 2, It is characterized in that The low-pass filter circuit includes a resistor R9 and a capacitor C2, one end of the resistor R9 serves as the input end of the low-pass filter circuit, the other end of the resistor R9 is connected to the input end of the isolation circuit and one end of the capacitor C2, and the other end of the capacitor C2 is connected to the other end of the resistor R6.

4. The AC cross-current suppression circuit of the station DC system according to claim 3, It is characterized in that The isolation circuit includes an isolation optocoupler U1, a capacitor C1, a capacitor C3, a capacitor C4, a capacitor C5 and an inductor L1. The positive input and negative input of the isolation optocoupler U1 serve as input ends of the isolation circuit, the positive input of the isolation optocoupler U1 is connected to the other end of the resistor R9, and the negative input of the isolation optocoupler U1 is connected to the other end of the resistor R6; the capacitor C1 is connected between the VDD1 pin and the negative input of the isolation optocoupler U1; the inductor L1 is connected between the voltage VCC+5V and the VDD2 pin of the isolation optocoupler U1; the capacitor C4 is connected between the VDD2 pin of the isolation optocoupler U1 and ground; the positive output and negative output of the isolation optocoupler U1 serve as output ends of the isolation circuit; the capacitor C3 is connected between the positive output of the isolation optocoupler U1 and ground, and the capacitor C5 is connected between the negative output of the isolation optocoupler U1 and ground.

5. The AC cross-current suppression circuit of the station DC system according to claim 4, It is characterized in that The voltage amplifier circuit includes a resistor R3, a resistor R5, a resistor R10, a resistor R11, a resistor R12, a resistor R13, a resistor R14, a capacitor C6, a capacitor C7, a capacitor C8, a capacitor C9, an inductor L2 and an operational amplifier U3A; one end of the resistor R10 and one end of the resistor R11 serve as input ends of the voltage amplifier circuit, one end of the resistor R10 is connected to the positive output of the isolation optocoupler U1, and one end of the resistor R11 is connected to the negative output of the isolation optocoupler U1; the other end of the resistor R10 is connected to the in-phase input end of the operational amplifier U3A, and the other end of the resistor R11 is connected to the inverting input end of the operational amplifier U3A. Input terminal; resistor R12 is connected between the non-inverting input terminal of the operational amplifier U3A and ground; the reference voltage is input to the non-inverting input terminal of the operational amplifier U3A through resistor R13; capacitor C9, resistor R3 and resistor R5 are respectively connected between the inverting input terminal and the output terminal of the operational amplifier U3A; the power supply voltage VCC3.3 is input to the positive voltage of the operational amplifier U3A through the inductor L2; capacitor C6 and capacitor C7 are connected between the positive voltage of the operational amplifier U3A and ground; capacitor C6 is a polar capacitor, and the negative end is connected to the ground; the output terminal of the operational amplifier U3A serves as the output terminal of the voltage amplifier circuit.

6. The AC cross-current suppression circuit of the station DC system according to claim 1, It is characterized in that The output filter circuit includes a resistor R14 and a capacitor C8, one end of the resistor R14 is connected to the output end of the voltage amplifier circuit, the other end of the resistor R14 is connected to one end of the capacitor C8 and serves as a voltage sampling end; the other end of the capacitor C8 is grounded.

7. The AC cross-current suppression circuit of the station DC system according to claim 1, It is characterized in that The basis for determining the specific location of the leakage current is: If U+>0 or U->0, it indicates that there is an AC current fault in the DC bus; If U R1n +>0, indicating that the positive electrode of the nth branch is connected to the alternating current; If U R2n +>0, indicating that the negative pole of the nth branch is connected to the alternating current; Among them, U+ is the AC voltage of the DC bus positive to the ground, U- is the AC voltage of the DC bus negative to the ground, and U R1n + is the voltage across the positive AC resistance of the nth branch, U R2n + is the voltage across the negative AC resistor of the nth branch.

8. An AC cross-current suppression device for a station DC system, It is characterized in that It comprises a housing, a printed circuit board and the AC current suppression circuit according to any one of claims 1 to 7, wherein the AC current suppression circuit is sealed in the housing after being soldered on the printed circuit board.

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