A relay normally open contact sticking detection circuit for SVG

Abstract

This invention discloses a relay normally open contact sticking detection circuit for SVG, including a central processing unit, a relay driving unit, a relay, a 12V power supply, and a coil current sampling and comparison unit. The central processing unit is connected to the relay driving unit and the coil current sampling and comparison unit via a pulse signal. The relay driving unit amplifies the pulse signal and applies it to the relay coil. The relay coil is connected to the 12V power supply. The relay driving unit is connected to the coil current sampling and comparison unit. This invention utilizes the characteristic of the relay coil to detect relay normally open contact sticking after the relay is disconnected. If the normally open contact is sticking, the coil core forms a closed magnetic circuit; if the relay is normally disconnected, the coil core forms an open magnetic circuit. The relay coil's characteristic is a series connection of inductance and resistance, and the inductance value after the normally open contact is sticking is larger than that under normal disconnection conditions. This feature of the relay coil is fully utilized for relay normally open contact sticking detection, making it suitable for SVG.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a relay normally open contact adhesion detection circuit for SVG. Background Technology

[0002] Before the SVG is connected to the grid, the bus capacitor has zero voltage because it has no energy source. Therefore, after the SVG is switched on and connected to the grid, a resistor is usually connected in series between the inverter circuit and the grid to precharge the bus capacitor through a soft-start resistor. After the bus capacitor is precharged to a higher voltage, the relay in the main circuit closes, bypassing the soft-start resistor and starting normal operation.

[0003] In practical applications, if a short circuit or lightning strike occurs in the power grid, the SVG (Inverter Var Generator) may generate a significant inrush current, causing the relays in the main circuit to stick. Although the relays may stick, the IGBT disconnection circuit in the inverter circuit will not exhibit any abnormalities after the SVG is shut down. Furthermore, due to cost constraints, single-pole single-throw relays are typically used, lacking additional contacts for relay status detection. In this situation, after the SVG trips and disconnects the grid, the voltage of the bus capacitor gradually drops to zero volts. When the SVG is reconnected to the grid, the relay sticking can cause a short circuit in the power grid, and the inrush current can damage the IGBTs in the inverter circuit, leading to very serious consequences.

[0004] Patent document CN114035034A (application number: CN202111288754.8) discloses a method and apparatus for diagnosing relay sticking based on coil current characteristics. The method includes at least one current monitoring device and a relay control switch disposed in the relay control circuit. When pre-charge side sticking is determined by voltage method, the main positive relay, main negative relay, and pre-charge relay are disconnected via the relay control switch. Then, the pre-charge side main relay is closed, and the coil current of the pre-charge side main relay is monitored by the current monitoring device. The sticking status of the pre-charge side main relay is determined based on the coil current: if not sticking, the pre-charge relay is determined to be sticking; if sticking, the power supply to the pre-charge side main relay coil is disconnected, the pre-charge relay is closed, and the coil current of the pre-charge relay is monitored by the current monitoring device. The sticking status of the pre-charge relay is determined based on the coil current. In short, patent document CN114035034A checks for sticking by detecting a sudden change in coil current at the moment the relay contacts close. However, the general method for checking for relay sticking involves detecting the voltage and current in the main contact or auxiliary contact circuit to determine whether there is sticking. This technology is not applicable to relays in SVG. Summary of the Invention

[0005] To address the shortcomings of the prior art, this invention provides a relay normally open contact adhesion detection circuit for SVG, which can safely and reliably distinguish between pre-charge relay adhesion and main circuit relay adhesion, thereby more accurately and thoroughly locating adhesion faults in SVG and improving the reliability of SVG.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A relay normally open contact adhesion detection circuit for SVG includes a central processing unit, a relay driving unit, a relay, a 12V power supply, and a coil current sampling and comparison unit.

[0008] The central processing unit is connected to the relay driving unit and the coil current sampling and comparison unit via pulse signals;

[0009] The relay driving unit amplifies the pulse signal and applies it to the relay coil;

[0010] The relay coil is connected to a 12V power supply;

[0011] The relay drive unit is connected to the coil current sampling and comparison unit.

[0012] Furthermore, the central processing unit is an FPGA (Field-Programmable Gate Array).

[0013] Furthermore, the relay driving unit consists of a MOSFET and a diode;

[0014] The gate connection pulse signal of the MOSFET;

[0015] The drain of the MOSFET is connected to the anode of the diode and one end of the relay coil;

[0016] The source of the MOSFET is connected to the coil current sampling and comparison unit;

[0017] The cathode of the diode is connected to a 12V power supply.

[0018] Furthermore, the relay driving unit applies the pulse signal generated by the central processing unit to the coil of the relay after it is amplified by a MOSFET.

[0019] The diode provides a current path for coil energy decay when the MOSFET is turned off.

[0020] Furthermore, the coil current sampling and comparison unit includes a current sampling resistor R, a comparator Q1, a NOR gate Q2, and a NOR gate Q3;

[0021] One end of the sampling resistor R is connected to the source of the MOSFET and the positive terminal of the comparator Q1, and the other end is grounded;

[0022] The negative terminal of the comparator Q1 is connected to the reference voltage;

[0023] The enable terminal of the comparator Q1 is connected to a pulse signal;

[0024] The input of the NOR gate Q2 is connected to the output of the comparator Q1 and the output of the NOR gate Q3, and the output of the NOR gate Q2 is connected to the input of the NOR gate Q3.

[0025] The input of the NOR gate Q3 is connected to the central processing unit (CPU), and the output of the NOR gate Q3 is also connected to the CPU.

[0026] Compared with the prior art, the advantages of the present invention are:

[0027] After a relay trips, if the normally open contact sticks, the magnetic core of its coil forms a closed magnetic circuit. If the relay trips normally, the magnetic core of its coil forms an open magnetic circuit. The coil characteristic of a relay is a series connection of inductance and resistance. Furthermore, the inductance value of a relay after its normally open contact sticks will be greater than that under normal tripping conditions. Utilizing this characteristic of the relay coil for detection of normally open contact sticking is particularly suitable for SVG. Attached Figure Description

[0028] Figure 1 This is the topology of a grid-connected inverter;

[0029] Figure 2 The schematic diagram of the detection circuit;

[0030] Figure 3 The coil with open magnetic circuit is opened for contact disconnection;

[0031] Figure 4 The coil's closed magnetic circuit is formed by the contact points sticking together;

[0032] Figure 5 The waveforms are pulse voltage waveforms and relay coil current waveforms under normal and abnormal conditions. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1, as Figure 1 As shown:

[0035] A relay normally open contact adhesion detection circuit for SVG includes a central processing unit, a relay driving unit, a relay, a power supply, and a coil current sampling and comparison unit.

[0036] The central processing unit is connected to the relay driving unit and the coil current sampling and comparison unit respectively. The two ends of the relay coil are connected to the 12V power supply and the relay driving unit respectively. The relay driving unit is connected to the coil current sampling and comparison unit.

[0037] The central processing unit generates pulse signals. These pulse signals are amplified by the relay drive unit and become pulse voltages applied to the relay coil. The volt-second value of the pulse voltage must not be large enough to trigger the relay contacts. The pulse signals are also sent to the coil current sampling and comparison unit as the enable signal for the coil current sampling and comparison unit.

[0038] To achieve the above functions, the central processing unit is implemented using a programmable gate array (FPGA).

[0039] The relay drive unit consists of a MOSFET and a diode. The gate of the MOSFET is connected to the central processing unit (where the pulse signal is input), the drain of the MOSFET and the anode of the diode are connected to one end of the relay coil, the source of the MOSFET is connected to the coil current sampling and comparison unit, and the cathode of the diode is connected to the 12V power supply.

[0040] The relay drive unit applies the pulse signal generated by the central processing unit to the relay coil after being amplified by the MOSFET, and at the same time uses a diode to provide a current path for coil energy attenuation when the MOSFET is turned off.

[0041] The coil current sampling and comparison unit includes a current sampling resistor R, a comparator Q1, and NOR gates Q2 and Q3. One end of the sampling resistor R is connected to the source of the MOSFET and then to the positive terminal of the comparator Q1. The other end of the sampling resistor R is grounded. The negative terminal of the comparator Q1 is connected to the reference voltage. The enable terminal of the comparator Q1 is connected to a pulse signal. One input terminal of the NOR gate Q2 is connected to the output terminal of the comparator Q1. The output terminals of the NOR gate Q3 and Q2 are connected to one input terminal of the NOR gate Q3. The other input terminal of the NOR gate Q3 is connected to the central processing unit as a reset signal. The signal output of the NOR gate Q3 is sent to the central processing unit as a fault signal.

[0042] Operating principle:

[0043] The coil current sampling and comparison unit is enabled by a pulse signal from the central processing unit. It converts the current signal of the relay coil into a voltage signal through a sampling resistor and compares it with a reference voltage. If the relay contacts are not stuck, the magnetic circuit of the relay coil is an open magnetic circuit with a small inductance value. After the applied pulse voltage, the peak current of the coil is greater than the current after the relay contacts are stuck. The output of the comparator will flip from low to high. After the comparator output signal flips, it will be latched by a latching circuit built with two NOR gates and sent back to the central processing unit. If the central processing unit does not receive a high level feedback after sending a pulse signal, it is considered that the relay contacts are stuck.

[0044] It mainly utilizes the fact that when the normally open contacts of a relay stick together, the magnetic circuit of the coil changes, thereby causing a change in the inductance of the coil. The fault of relay contact sticking is judged by a pulse voltage with a small volt-second value.

[0045] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A relay normally open contact sticking detection circuit for an SVG, characterized by, The relay drive unit, the relay, the 12V power supply and the coil current sampling comparison unit are connected with the central processing unit. The central processing unit is connected with the relay drive unit and the coil current sampling comparison unit through pulse signals. The relay drive unit amplifies the pulse signals and then applies them to the coil of the relay. The coil of the relay is connected with the 12V power supply. The relay drive unit is connected with the coil current sampling comparison unit. The central processing unit is a FPGA. The relay drive unit is composed of a MOSFET and a diode. The gate of the MOSFET is connected with the pulse signals. The drain of the MOSFET is connected with the anode of the diode and one end of the coil of the relay. The source of the MOSFET is connected with the coil current sampling comparison unit. The cathode of the diode is connected with the 12V power supply. The relay drive unit amplifies the pulse signals generated by the central processing unit through the MOSFET and then applies them to the coil of the relay. The diode provides a current channel for the coil energy attenuation when the MOSFET is turned off. The coil current sampling comparison unit includes a current sampling resistor R, a comparator Q1, an NOR gate Q2 and an NOR gate Q3. One end of the sampling resistor R is connected with the source of the MOSFET and the positive electrode of the comparator Q1, and the other end is grounded. The negative electrode of the comparator Q1 is connected with a reference voltage. The enable end of the comparator Q1 is connected with the pulse signals. The input end of the NOR gate Q2 is connected with the output end of the comparator Q1 and the output end of the NOR gate Q3, and the output end of the NOR gate Q2 is connected with the input end of the NOR gate Q3. The input end of the NOR gate Q3 is connected with the central processing unit, and the output end of the NOR gate Q3 is connected with the central processing unit.

Images