A protection circuit for preventing the expansion of an AC side fault of a controllable rectifier

Abstract

This invention discloses a protection circuit to prevent the escalation of AC side faults in a controllable rectifier, comprising a current transformer, a three-phase bridge rectifier ZLQ, a Zener diode D1, an isolation diode D2, a switching transistor T1, and a transistor T2. The current transformer, the controllable rectifier, ZLQ, and the negative terminal of D1 are connected in sequence. The positive terminal of D1 is connected to the base (b) of T1 via R6 in series. The collector (c) of T1 is connected to the base (b) of T2 via R10 in series. The collector (c) of T2 is connected to a reset button via R9 in series. The collector (c) of T1 is also connected to a reset button via R7 in series. The collector (c) of T1 is connected to the negative terminal of D2 via R8 in series. The positive terminal of D2 is connected to a high-level DC short-circuit protection circuit. The voltage at the collector of T1 is transmitted to the main control system as a short-circuit signal on the AC side of the rectifier. The voltage between R8 and D2 serves as the blocking level for the thyristor pulse circuit. This invention can promptly detect and effectively block the thyristor trigger pulse signal when the AC side of the controllable rectifier is short-circuited.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically to a protection circuit for preventing the spread of faults on the AC side of a controllable rectifier. Background Technology

[0002] High-power controllable rectifiers are widely used in industrial and mining enterprises, power companies, and rail transportation. These high-power rectifiers use a large number of expensive high-power semiconductor devices, thyristors. These devices are critical components in controllable rectifiers and must operate safely and reliably. However, most high-power rectifier applications involve overloads and electrical shocks that exceed normal operating conditions, easily causing thyristor breakdown and damage. Once one thyristor breaks down, a short circuit caused by a metal object, or a short circuit caused by conductive dust or water arcing on the device surface, if the fault cannot be isolated in time, will lead to successive breakdowns of thyristors, resulting in significant equipment and production losses. To protect these critical devices, high-power fast-acting fuses are often connected in series in the thyristor circuit. This prevents overload damage to the thyristors and isolates faulty circuits. Currently, high-power rectifiers adopt this solution.

[0003] While using high-power thyristor branches with series fast-acting fuses to protect thyristors and isolate faults is effective, it is also a last resort. Although high-capacity fast-acting fuses have a significant cost advantage over thyristors, their price still constitutes a considerable portion of the equipment investment cost. Their large size leads to complex structures, occupying a lot of space, making assembly and replacement inconvenient. Furthermore, fast-acting fuses are disposable; once they blow, they need to be replaced, which is troublesome and costly. Additionally, fast-acting fuses themselves are current conductors, consuming electrical energy and generating heat under normal operating conditions, which negatively impacts the equipment's operating environment and hinders energy conservation and consumption reduction. Summary of the Invention

[0004] 1. The technical problem to be solved:

[0005] To address the aforementioned technical problems, this invention provides a protection circuit to prevent the escalation of AC-side faults in a controllable rectifier. It can block or maintain the thyristor pulse control signal under three conditions: normal operation, AC-side short circuit, and DC-side short circuit. When an AC short circuit occurs, the thyristor control pulse is blocked, preventing other thyristors from conducting. This eliminates the short-circuit fault current on the rectifier side, preventing it from freewheeling and thus protecting the rectifier. Simultaneously, the sampling circuit outputs a rectifier AC-side short-circuit fault message to the system main control board. Upon receiving this signal, the main control board immediately runs in rectifier fault protection mode and issues another pulse blocking signal and AC-side fault message, achieving dual protection. To distinguish the DC-side short circuit condition, the controllable rectifier must operate in rectifier overload mode to shunt the short-circuit current flowing through the IGBT freewheeling diode in the inverter, thus protecting the IGBT.

[0006] 2. Technical Solution:

[0007] A protection circuit for preventing the spread of AC side faults in a controllable rectifier, used to protect a bidirectional converter in rail transit; characterized in that: the controllable rectifier in the bidirectional converter is configured to expand the DC output capability, and in rectification mode, the controllable rectifier and the converter are connected in parallel to supply power to the DC load; the main control system of the bidirectional converter coordinates the output of the controllable rectifier and the converter by controlling the trigger pulses of the thyristors of the controllable rectifier.

[0008] In this application, the bidirectional converter is a high-power power electronic device used in rail transit to provide DC power for driving electric locomotives and to feed back the electrical energy converted from mechanical energy during the locomotive's braking process into the AC power grid system. The controllable rectifier is configured to expand the DC output capacity of the converter. In rectification mode, the controllable rectifier and the converter are connected in parallel to supply power to the DC load. The main control system of the bidirectional converter coordinates the output of the controllable rectifier and the converter by controlling the trigger pulses of the thyristors of the controllable rectifier.

[0009] The protection circuit includes a three-phase current transformer, a three-phase bridge rectifier ZLQ, a Zener diode D1, an isolation diode D2, a switching transistor T1, and a transistor T2. The specific connection method is as follows: the three-phase currents are connected to the AC input terminals of the controllable rectifier through three current transformers, and the output terminals of the transformers are connected to the three AC input terminals of the three-phase bridge rectifier ZLQ. The output terminal of the three-phase bridge rectifier ZLQ is connected to the negative terminal of the Zener diode D1. The positive terminal of the Zener diode D1 is connected in series with a resistor R6 and then to the base of the switching transistor T1. The collector of transistor T1 is connected in series with resistor R10 and then to the base of transistor T2; the collector of transistor T2 is connected in series with resistor R9 and then to the reset button; the collector of switching transistor T1 is also connected in series with resistor R7 and then to the reset button; the collector of switching transistor T1 is connected in series with resistor R8 and then to the negative terminal of isolation diode D2; the positive terminal of isolation diode D2 is connected to the high level of DC short-circuit protection sent from the main control system; the voltage signal of the collector of switching transistor T1 is transmitted to the main control system as the AC side short-circuit signal of the rectifier;

[0010] The voltage between resistor R8 and isolation diode D2 serves as the blocking level for the thyristor pulse circuit. After receiving an abnormally high current signal on the AC side of the rectifier, the main control system determines whether there is a short circuit on the DC side and decides whether to send a blocking level signal to the thyristor trigger pulse circuit of the controllable rectifier.

[0011] The voltage between resistor R8 and isolation diode D2 serves as the blocking level for the thyristor pulse circuit. The main control system receives the blocking level signal and sends it to the thyristor trigger pulse circuit of the controllable rectifier.

[0012] Furthermore, a feedback resistor R11 is connected in parallel between the base of the switching transistor T1 and the emitter of the transistor T2; the three-phase current transformer is grounded after being connected in series with conversion resistors R1, R2, and R3 at its output terminal; the output terminal of the three-phase bridge rectifier ZLQ is connected in parallel with a resistor R4 and then connected to a Zener diode D1; a resistor R5 is connected in series between the positive terminal of the Zener diode D1 and the negative output terminal of the three-phase bridge rectifier ZLQ.

[0013] 3. Beneficial effects:

[0014] This solution eliminates the need for expensive fast-acting fuses in the controlled rectifier, simplifying its structure and reducing its size. It also reduces internal heat generation, maintenance costs, and workload. More importantly, in the event of a short-circuit fault on the AC side of the controlled rectifier, the protection circuit can detect it promptly and effectively block the thyristor trigger pulse signal. The blocking voltage Uk is only about 0.2V (compared to about 21V during normal operation, excluding fault conditions). Simultaneously, it can promptly send an AC side short-circuit fault signal to the main controller. Based on the selected thyristor I in the bidirectional converter auxiliary controlled rectifier...2 The measured t-value and actual short-circuit current magnitude, from the start of the short-circuit fault to the effective time of the blocking voltage Uk, are no more than 10ms, which is less than half of the surge protection time of the thyristor. This is sufficient to protect the thyristor and eliminate the short circuit. During the DC-side short-circuit test, the blocking signal is replaced by a high-level DC-side short-circuit protection signal issued by the main control unit, and the rectifier can still work normally. The measured data shows that the rectifier can shunt more than 80% of the short-circuit peak current that was originally borne by the IGBT freewheeling diode in the converter, effectively protecting the IGBT from damage by the short-circuit surge current. Attached Figure Description

[0015] Figure 1 The schematic diagram shows the protection circuit of this invention for implementing rectifier fault short-circuit protection.

[0016] Figure 2 The diagram illustrates the current distribution principle for a DC-side short circuit in a bidirectional converter using the protection circuit of this invention. Detailed Implementation

[0017] The present invention will now be described in detail with reference to the accompanying drawings.

[0018] As attached Figure 1 As shown, this solution provides a low-cost AC-side current sampling signal processing and transmission to the main control board. It blocks or maintains the rectifier thyristor pulse control signal under three conditions: normal operation, AC-side short circuit, and DC-side short circuit. When an AC short circuit fault occurs, the thyristor control pulse is blocked, preventing other thyristors from conducting. This eliminates the short-circuit fault current from freewheeling on the rectifier side, preventing thyristor breakdown due to short-circuit current impact and eliminating the fault current (preventing fault escalation), thus protecting the rectifier. Simultaneously, the sampling circuit also outputs a rectifier AC-side short-circuit fault message to the main control board. Upon receiving this signal, the main control board immediately runs in rectifier fault protection mode and reissues the pulse blocking signal and rectifier AC-side fault message, achieving dual protection. To differentiate the situation under DC-side short circuit conditions, the controllable rectifier must operate in rectifier overload mode to shunt the short-circuit current flowing through the IGBT freewheeling diode in the inverter, thus protecting the IGBT.

[0019] To address the aforementioned technical problems, the present invention has adopted the following measures.

[0020] First, sampling current transformers CTa, CTb, and CTc are added to the three-phase AC input side of the rectifier. Small-value, high-power resistors (greater than 20W) are connected across the output terminals of these current transformers. Voltage waveforms consistent with the current waveform are generated across these sampling resistors. This three-phase voltage, proportional to the three-phase AC current, serves as the input voltage of the three-phase bridge rectifier ZLQ. The sampling rectifier ZLQ outputs a DC voltage. Under normal operating conditions of the controllable rectifier, its DC output is relatively small, only a few volts, which cannot reach the stable voltage value Uz of the Zener diode D1. The switching transistor T1 remains off, transistor T2 remains saturated, and Uk remains high. The controllable rectifier pulse can normally trigger thyristors KP1~KP6. However, if a short circuit occurs on the AC side of the controllable rectifier, such as thyristor damage, short circuit caused by metal objects, or short circuit caused by conductive dust arcing on the device surface, a large short-circuit current will be generated. This short-circuit current is much larger than the normal operating current, resulting in a large voltage signal across the sampling resistor of the current transformer. This voltage, output through ZLQ, generates a DC voltage significantly higher than normal, exceeding the Uz value of D1. This causes T1 to saturate and conduct, bringing its output Uk close to 0 volts. This voltage level controls the enable terminals of all thyristors in the controlled rectifier, blocking the thyristor trigger pulse signals and keeping all thyristors in a blocked state, thus protecting the thyristor devices. Simultaneously, the saturation conduction of transistor T1 causes T2 to exit the conducting state and enter the cutoff state. The feedback voltage Uf becomes high, and through the feedback resistor R11, it further maintains the saturation conduction of transistor T1, keeping it in this state. Only after the short-circuit fault has been resolved and normal operation of the controlled rectifier needs to be restored, can the blocking pulse signal be cleared by pressing the reset button, causing transistor T1 to exit the saturation conduction state and return to the cutoff state.

[0021] In the bidirectional converter project for rail transit, AC fault protection does not affect the controllable rectifier's ability to shunt the short-circuit current of the IGBT freewheeling diodes in the converter under DC-side short-circuit conditions. Although a DC-side short circuit will generate a huge output current in the controllable rectifier, causing T1 to saturate and conduct, when the main control detects a DC-side short circuit, it will send a high-level signal to this protection circuit. Due to the effect of the isolation diode D2, the blocking signal is clamped at a high level, thus rendering the blocking signal ineffective. The controllable rectifier can utilize its own overload capacity to temporarily share most of the DC-side short-circuit current. Once the control system detects a DC-side short circuit, it will quickly put the DC power supply system into short-circuit fault handling mode, thereby ensuring the safety of the DC power supply equipment. Specific implementation examples:

[0023] In practical applications, the rated voltage of the controllable rectifier is 1500V and the rated current is 2000A; the thyristors KP1 to KP6 are usually of model KK28F45N, with a non-repetitive on-state current of 45KA. 2 t is 10244; a three-phase rectifier bridge with low tube voltage drop is selected for the sampling rectifier.

[0024] As attached Figure 1 As shown, in this scheme, three CTA, CTB, and CTC current transformers (2000 / 5) are installed on the AC input circuit of the controllable rectifier to detect the AC input current of the controllable rectifier. The three-phase current transformers are selected with an output power greater than 15 watts. The conversion resistors R1, R2, and R3 are 0.5Ω / 20W resistors, R4, R5, and R6 are 1kΩ / 0.5W resistors, R7 is 1kΩ / 1W, R9 is 4.7kΩ / 1W, and R8, R10, and R11 are 10kΩ / 0.5W. T1 and T2 are 3DK series transistors, the Zener diode D1 is a 6-8V Zener diode, and the isolation diode D2 is a 1N1007. VCC is 24V.

[0025] 1. In normal rectification mode: the three-phase AC input current is less than 1800A, the output current of the three current transformers is between 0-4.5A, and the DC output voltage of the three-phase bridge rectifier ZLQ is around 4V. Considering the voltage drop of the internal transistors of the bridge rectifier, no current flows through the Zener diode D1, T1 is cut off, T2 is saturated and conducting, the collector voltage Uk of T1 is greater than 20V, and the collector voltage Uf of T2 is around 0.2V. When the enable control terminal of the controllable rectifier is high, the trigger pulse can trigger the thyristors, and the rectifier outputs normally.

[0026] 2. When a fault short circuit occurs on the AC side of the rectifier: for example, if one of the thyristors in KP1 to KP6 breaks down, a short circuit is caused by a metal component, or a surface arc short circuit is caused by water or conductive dust on the surface of the device, the AC current transformer will detect the fault short circuit current, causing it to generate an output current much greater than the rated current of 5A. This current is converted into voltage by the resistor, causing the output of the three-phase rectifier bridge ZLQ to rise instantaneously. The high output voltage is greater than the stable voltage Uz of D1, causing D1 to conduct, thus turning on T1 and turning off T2. The output voltage becomes high and turns on. Current is injected into T1 through feedback resistor R11, causing T1 to quickly saturate and conduct. At this time, Uk is approximately 0.2V, and the trigger pulse signals of KP1 to KP6 are blocked through R8. Simultaneously, a low-level blocking pulse signal is sent to the main control board. The main control program immediately enters the AC side fault short-circuit handling mode, and the software also sends a blocking pulse signal, forming dual protection. KP1 to KP6 are then turned off, the fault circuit is blocked, and the fault current cannot pass through the thyristor, thus effectively protecting the thyristor and other devices in the DC circuit. During this period, T1 always outputs a low level, regardless of whether the AC side fault short circuit disappears. Only after the fault is handled and the reset button is pressed can the blocking signal be released, allowing the controllable rectifier to restart.

[0027] 3. DC side short circuit condition: see attached. Figure 2 The protection circuit of this scheme is explained in conjunction with the DC-side short-circuit protection of the bidirectional converter. In the bidirectional converter circuit, the converter and the output of the controllable rectifier are connected in parallel to supply power to the DC-side load. Since the converter has a high-power IGBT, although the system will promptly block the IGBT trigger pulse when a DC-side short-circuit fault is detected, its internal freewheeling diode will remain in rectification mode because the DC voltage on the DC side is much smaller than the AC voltage. The freewheeling diode carries a large short-circuit current and overcurrent breakdown, directly damaging the expensive IGBT component. Therefore, when a DC-side short circuit occurs, the controllable rectifier needs to continue operating, making full use of the overload capacity of the thyristor (which is much greater than that of the freewheeling diode in the IGBT) to share most of the current originally flowing through the freewheeling diode in the converter. To protect the IGBT from short-circuit current, the protection circuit uses a high-level short-circuit protection signal from the DC side, via diode D2, to force the low-level signal UK (which blocks the thyristor pulse) that might occur under DC short-circuit conditions. This ensures the controllable rectifier operates under high output short-circuit current conditions, protecting the IGBT and preventing the DC side fault from spreading to the bidirectional converter, thus providing excellent protection. Another function of diode D2 is to isolate the blocking signal UK from the main control board under non-DC side short-circuit fault conditions, ensuring the protection circuit can properly block the pulse under AC side short-circuit conditions, protecting the controllable rectifier thyristor and preventing further fault escalation.

[0028] Although the present invention has been disclosed above with reference to preferred embodiments, these are not intended to limit the invention. Any person skilled in the art can make various changes or modifications without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention should be defined by the scope of the claims of this application.

Claims

1. A protection circuit for preventing the spread of faults on the AC side of a controllable rectifier, used to protect a bidirectional converter in rail transit; characterized in that: The controllable rectifier in the bidirectional converter is configured to expand the DC output capability. In rectification mode, the controllable rectifier and the converter are connected in parallel to supply power to the DC load. The main control system of the bidirectional converter coordinates the output of the controllable rectifier and the converter by controlling the trigger pulse of the thyristor of the controllable rectifier. The protection circuit includes a three-phase current transformer, a three-phase bridge rectifier ZLQ, a Zener diode D1, an isolation diode D2, a switching transistor T1, and a transistor T2. The specific connection method is as follows: the three-phase currents are connected to the AC input terminals of the controllable rectifier through three current transformers, and the output terminals of the transformers are connected to the three AC input terminals of the three-phase bridge rectifier ZLQ. The output terminal of the three-phase bridge rectifier ZLQ is connected to the negative terminal of the Zener diode D1. The positive terminal of the Zener diode D1 is connected in series with a resistor R6 and then to the base of the switching transistor T1. The collector of transistor T1 is connected in series with resistor R10 and then to the base of transistor T2; the collector of transistor T2 is connected in series with resistor R9 and then to the reset button; the collector of switching transistor T1 is also connected in series with resistor R7 and then to the reset button; the collector of switching transistor T1 is connected in series with resistor R8 and then to the negative terminal of isolation diode D2; the positive terminal of isolation diode D2 is connected to the high level of DC short-circuit protection sent from the main control system; the voltage signal of the collector of switching transistor T1 is transmitted to the main control system as the AC side short-circuit signal of the rectifier; The voltage between resistor R8 and isolation diode D2 serves as the blocking level for the thyristor pulse circuit. After receiving an abnormally high current signal on the AC side of the rectifier, the main control system determines whether there is a short circuit on the DC side and decides whether to send a blocking level signal to the thyristor trigger pulse circuit of the controllable rectifier.

2. The protection circuit for preventing the spread of AC side faults in a controllable rectifier according to claim 1, characterized in that: A feedback resistor R11 is connected in parallel between the base of the switching transistor T1 and the emitter of the transistor T2; the three-phase current transformer is grounded after being connected in series with conversion resistors R1, R2, and R3 at its output terminal; the output terminal of the three-phase bridge rectifier ZLQ is connected to the Zener diode D1 after being connected in parallel with resistor R4; a resistor R5 is connected in series between the positive terminal of the Zener diode D1 and the negative output terminal of the three-phase bridge rectifier ZLQ.

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