Composite protection low voltage circuit breaker

Abstract

The present invention discloses a composite protection low-voltage circuit breaker, comprising a trip unit, a zero-sequence induction coil, and a composite protection circuit. The composite protection circuit is connected to the phase output and the zero-phase input of the low-voltage circuit breaker. The composite protection circuit requires only a high-voltage trigger diode and a resistor to implement overvoltage protection. The high-voltage trigger diode, a mature, highly reliable passive component, significantly improves reliability compared to overvoltage protection devices that utilize extensive active detection circuitry. The present invention achieves islanding protection at a very low cost and solves the problem of false tripping of leakage circuit breakers.

Description

Technical Field

[0001] The present invention relates to the field of power protection equipment, in particular to a composite protection low-voltage circuit breaker. Background Art

[0002] With the advancement of power electronics technology, an increasing number of small microgrids with distributed power sources are being deployed. As a fundamental component of low-voltage distribution network protection, how to adapt miniature circuit breakers to these systems is a pressing issue. Microgrid systems contain a large number of power electronic converters. Due to the special requirements of converter equipment and their sensitivity to power system disturbances, they have unique fault protection requirements.

[0003] Power electronic devices have a poor tolerance for overvoltage stress, and relying solely on the converter's own protection can sometimes have significant limitations. Some single-phase microgrid experimental systems have experienced converter burnout due to zero-phase open-circuit overvoltage. In such cases, external devices, such as miniature circuit breakers, are required to quickly clear the fault and reliably protect the converter.

[0004] In addition, all distributed power converters have islanding effect protection functions, but in actual applications, the problem of delayed operation often occurs. If auxiliary backup protection for islanding effect is achieved in a simple way with the help of external circuit breakers, the safety of the microgrid system can be further improved. Islanding effect protection is indispensable for the terminal power grid containing distributed power sources. Many new energy inverter units have achieved good results by combining active and passive islanding effect protection schemes. However, for low-voltage and small-capacity microgrid systems, such as household photovoltaic systems, due to the relatively simple islanding protection detection of the inverter itself, problems such as slow shutdown after external power grid failure often occur. Moreover, since its shutdown relies solely on the shutdown of the converter, it cannot achieve isolation of the mechanical disconnection point of the switching device, so there are still certain hidden dangers.

[0005] At the same time, since power electronic equipment uses a large number of anti-interference capacitors to the ground, the capacitive current accumulated by these capacitors will easily cause the leakage circuit breaker to trip incorrectly.

[0006] Therefore, there is an urgent need to provide a new type of low-voltage circuit breaker to solve the above problems. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a composite protection low-voltage circuit breaker that can achieve effective overvoltage protection and islanding protection.

[0008] In order to solve the above technical problems, the present invention adopts a technical solution as follows: providing a composite protection low-voltage circuit breaker, comprising a trip unit, a zero-sequence induction coil, and a composite protection circuit, wherein the composite protection circuit is connected to the output terminal of the phase line and the input terminal of the zero phase of the low-voltage circuit breaker;

[0009] The composite protection circuit includes a load resistor R1, a current limiting resistor R2, a resonant capacitor C, a smoothing capacitor C1, a resonant inductor L, a diode D, a thyristor SCR, and a high-voltage trigger diode SIDAC; one end of the current limiting resistor R2 is connected to the output end of the low-voltage circuit breaker phase line, and the other end is connected in series with the high-voltage trigger diode SIDAC, and the other end of the high-voltage trigger diode SIDAC is connected to the input end of the zero phase; the resonant capacitor C is connected in parallel with the resonant inductor, and then one end is connected in parallel with the current limiting resistor R2, and the other end is connected in parallel with the gate of the thyristor SCR; the load resistor R1 is connected in series with the negative electrode of the diode D, and then one end is connected in parallel with the gate of the thyristor SCR, and the other end is connected in parallel with the positive electrode of the thyristor SCR; the smoothing capacitor C1 is connected in parallel to both ends of the load resistor R1 and the diode D, and the connection point between the load resistor R1 and the diode D is connected to the input end of the zero phase.

[0010] In a preferred embodiment of the present invention, the resonant frequency of the resonant inductor L and the resonant capacitor C is between 52-58 Hz.

[0011] The beneficial effects of the present invention are:

[0012] (1) The present invention achieves island protection at a very low cost. The technical solution proposed by the present invention, by cleverly setting up a resonant circuit, directly drives the leakage protection release to trip under frequency offset by using the resonant circuit. It has the characteristics of simple structure and high reliability. At the same time, it can be achieved without using any chip control conditions, which is difficult to achieve with the existing technology.

[0013] (2) The present invention solves the problem of false tripping of leakage circuit breakers. The present invention adjusts the resonant frequency of the resonant circuit so that it is slightly inductive for the 50Hz power frequency. This results in a smaller inductive compensation current in the zero-sequence induction coil of the leakage circuit breaker, which can offset the capacitive leakage current, thereby avoiding the hidden danger of false tripping of the leakage circuit breaker.

[0014] (3) The present invention achieves an extremely high level of overvoltage protection. Existing overvoltage protection circuit breakers rely on a large number of electronic components, and their reliability and service life are average. The present invention only requires a high-voltage trigger diode and a resistor to achieve all the protection functions of the existing technology. The high-voltage trigger diode is a mature passive device with extremely high reliability. Compared with overvoltage protection devices designed with a large number of active detection circuits, it greatly improves reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a circuit diagram of the composite protection low-voltage circuit breaker of the present invention;

[0016] The components in the accompanying drawings are marked as follows: 1. trip unit, 2. zero-sequence induction coil, 3. composite protection circuit. DETAILED DESCRIPTION

[0017] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the protection scope of the present invention.

[0018] See also Figure 1 , embodiments of the present invention include:

[0019] A composite protection low-voltage circuit breaker includes a trip unit 1, a zero-sequence induction coil 2, and a composite protection circuit 3. The composite protection circuit 3 is connected to the phase line output and the zero-phase input of the low-voltage circuit breaker. The primary side of the zero-sequence induction coil 2 encircles the phase line and zero-phase of the circuit breaker, and the secondary side of the zero-sequence induction coil 2 is connected to the signal input of the trip unit 1. The signal output of the trip unit 1 is connected to the phase line and zero-phase input of the low-voltage circuit breaker.

[0020] The composite protection circuit 3 includes a load resistor R1, a current limiting resistor R2, a resonant capacitor C, a smoothing capacitor C1, a resonant inductor L, a diode D, a thyristor SCR, and a high-voltage trigger diode SIDAC; one end of the current limiting resistor R2 is connected to the output end of the low-voltage circuit breaker phase line, and the other end is connected in series with the high-voltage trigger diode SIDAC, and the other end of the high-voltage trigger diode SIDAC is connected to the input end of the zero phase; the resonant capacitor C is connected in parallel with the resonant inductor, and then one end is connected in parallel with the current limiting resistor R2, and the other end is connected in parallel with the gate of the thyristor SCR; the load resistor R1 is connected in series with the negative electrode of the diode D, and then one end is connected in parallel with the gate of the thyristor SCR, and the other end is connected in parallel with the positive electrode of the thyristor SCR; the smoothing capacitor C1 is connected in parallel to both ends of the load resistor R1 and the diode D, and the connection point between the load resistor R1 and the diode D is connected to the input end of the zero phase.

[0021] A SIDAC, also known as a high-voltage trigger diode, is a nonlinear two-terminal device with negative resistance. However, when the applied voltage across its terminals is below the turn-on voltage, the diode's resistance is extremely high, generating only microampere-level leakage current. When the applied voltage across its terminals exceeds the turn-on voltage, UBO, the diode rapidly transitions to a low-resistance conduction state. Its on-state voltage, UT, is very low, only around 1.5V, allowing it to instantly pass a large current. Once turned on, a SIDAC enters a self-latching state, shutting off only when the current flowing through it is interrupted or falls below the holding current. Its on-state mode resembles that of a varistor, while its off-state mode resembles that of a thyristor. Its unique volt-ampere characteristics enable the design of overvoltage detection circuits. SIDAC components have nanosecond response speeds, which promises to improve the detection of overvoltage anomalies.

[0022] Simply setting UBO, the SIDAC diode's operating threshold voltage, slightly above the peak trip voltage setting, ensures the SIDAC diode conducts immediately after an overvoltage anomaly. By setting the current-limiting resistor R2, the current after the SIDAC conducts is no less than the residual current protector's operating current. For example, if a circuit breaker needs to trip at a grid voltage of 250V, a K350 SIDAC diode can be used. When the grid voltage rises to 250V, the peak voltage is approximately 353V, exceeding the K350 diode's 350V conduction threshold. To ensure reliable operation of the residual current protector, assuming the operating current I = 50mA, then R2 = 2.5kΩ.

[0023] This invention utilizes frequency detection, the most fundamental islanding effect detection technology, to provide auxiliary protection against the islanding effect with the help of a residual current protector (RCP), thereby making islanding protection for low-voltage, small-capacity microgrid systems more reliable. The voltage across R1 varies with frequency, but when the loop Q value is large, a small frequency shift can significantly increase the voltage across R1. The voltage across R1 charges the smoothing capacitor C1 through diode D. When the voltage across R1 exceeds the gate trigger voltage of the thyristor (SCR), the charging cycle of several cycles triggers the thyristor to conduct, causing the zero-sequence transformer of the RCP to sense the unbalanced current and trip, thus providing frequency shift islanding effect protection. However, when the system frequency is normal, the admittance of the parallel resonant circuit is very small, and the voltage across R1 is much lower than the gate trigger voltage of the thyristor. The thyristor will not be mistakenly triggered to conduct. When there is a transient disturbance in the power grid, such as the opening and closing of the switch, a higher voltage may appear across R1 due to the transient process of the LC element. At that time, due to the energy storage buffering effect of C1, the thyristor will not be mistakenly triggered by a transient voltage of one or two cycles. Due to the large number of anti-interference ground capacitors used in power electronic equipment, the capacitive current accumulated by these capacitors will easily cause the leakage circuit breaker to trip incorrectly. The present invention adjusts the resonant frequency setting of the resonant circuit (that is, the inductive reactance parameters of the capacitor C and the inductor L are consistent) so that it is slightly inductive for the 50Hz power frequency. This allows a smaller inductive compensation current to be generated in the zero-sequence induction coil of the leakage circuit breaker, which can offset the capacitive leakage current, thereby avoiding the hidden danger of the leakage circuit breaker tripping incorrectly. Preferably, the resonant frequency of the resonant inductor L and the resonant capacitor C is between 52 and 58 Hz.

[0024] The present invention only requires a high-voltage trigger diode and a resistor to achieve all the protection functions of the existing technology. As a mature passive device with extremely high reliability, the high-voltage trigger diode greatly improves the reliability compared with the overvoltage protection device designed with a large number of active detection circuits.

[0025] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention's description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A composite protection low voltage circuit breaker, characterized in that: It includes a tripper, a zero-sequence induction coil, and a composite protection circuit, wherein the composite protection circuit is connected to the output end of the phase line of the low-voltage circuit breaker and the input end of the zero phase; The composite protection circuit includes a load resistor R1, a current limiting resistor R2, a resonant capacitor C, a smoothing capacitor C1, a resonant inductor L, a diode D, a thyristor SCR, and a high-voltage trigger diode SIDAC; one end of the current limiting resistor R2 is connected to the output end of the low-voltage circuit breaker phase line, and the other end is connected in series with the high-voltage trigger diode SIDAC, and the other end of the high-voltage trigger diode SIDAC is connected to the input end of the zero phase; the resonant capacitor C is connected in parallel with the resonant inductor, and then one end is connected in parallel with the current limiting resistor R2, and the other end is connected in parallel with the gate of the thyristor SCR; the load resistor R1 is connected in series with the negative electrode of the diode D, and then one end is connected in parallel with the gate of the thyristor SCR, and the other end is connected in parallel with the positive electrode of the thyristor SCR; the smoothing capacitor C1 is connected in parallel to both ends of the load resistor R1 and the diode D, and the connection point between the load resistor R1 and the diode D is connected to the input end of the zero phase.

2. The composite protection low voltage circuit breaker according to claim 1, characterized in that: The resonant frequency of the resonant inductor L and the resonant capacitor C is between 52 and 58 Hz.

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