A method for judging a fault of a relay on a direct current side of an energy storage converter

Abstract

The application discloses a kind of energy storage converter DC side relay fault judgment method including high voltage power-on stage fault diagnosis and operating state stage fault diagnosis.High voltage power-on fault includes: energy storage converter is in shutdown state and sends disconnecting drive signal to pre-charging relay K1, main relay K2;If the value of DC hall sensor A1 is greater than charging current threshold, then K2 is adhered;Otherwise, compare battery voltage V1, pre-charging capacitor voltage V2 to further judge whether K1 is adhered;If voltage does not satisfy corresponding condition, after sending drive signal to K1, compare V1, V2, if it does not satisfy preset condition, then K1 is disconnected;Otherwise, after sending drive signal to K2, if A1 does not satisfy preset condition, then judge K2 is disconnected.Operating state fault is to judge first that operating state is grid-connected or off-grid;If grid-connected, if A1 is less than threshold and V2 fluctuation value is greater than preset range, then K2 is disconnected;If off-grid, if A1 is less than preset value, then K2 is disconnected.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, specifically to a method for diagnosing faults in the DC-side relays of an energy storage converter. Background Technology

[0002] With the rapid increase in the proportion of new energy power generation, represented by photovoltaic and wind power, the stable operation of the power system faces severe challenges from the significant random fluctuations of wind and solar energy. To balance these fluctuations, electrochemical energy storage technology is widely used for energy storage and regulation of photovoltaic and wind power. Energy storage technology can help store the energy generated by photovoltaic and wind power to meet the needs of peak or off-peak load periods. Energy storage converters connect batteries and energy storage systems on the DC side and connect to the grid on the AC side, enabling bidirectional energy flow.

[0003] Energy storage converters typically have a DC main relay, a pre-charge relay, and a pre-charge resistor on the DC side. During startup and operation of the energy storage system, the converter controls the relays to pre-charge and energize its DC-side capacitors. Since high-voltage relays generally lack contact feedback signals, energy storage converters, in traditional methods, cannot directly know the relay status through feedback signals during DC-side power-on and grid-connected / off-grid operation.

[0004] The reliable closing and opening of the DC-side relays of the energy storage converter are crucial for the operation, shutdown, and fault protection of the energy storage system. Therefore, it is of great significance to perform independent fault diagnosis on the DC-side main relays and pre-charge relays of the converter. Summary of the Invention

[0005] 1. The technical problem to be solved: To address the aforementioned technical problems, this invention provides a method for diagnosing relay faults on the DC side of an energy storage converter. This method independently diagnoses potential relay sticking and open-circuit faults in the circuit during system power-on and operation, enabling real-time and accurate fault point identification, greatly facilitating system operation and maintenance and fault analysis.

[0006] 2. Technical Solution: A method for diagnosing DC-side relay faults in an energy storage converter is characterized by: setting up a DC-side pre-charge circuit on the DC side of the energy storage converter; the DC-side pre-charge circuit includes a battery pack and a pre-charge RC circuit; wherein a pre-charge capacitor C1 is connected in parallel with the DC side of the PCS energy storage converter, a pre-charge resistor R1 is connected in series with the pre-charge capacitor C1 and then connected to the battery pack via a pre-charge relay K1, and a main relay K2 and a DC Hall sensor A1 are connected in series in parallel with the pre-charge capacitor C1; the fault diagnosis method includes the following steps: Step 1: Set sampling points; the sampling points include: setting battery pack voltage sampling point V1 at both ends of the battery pack, and setting pre-charge capacitor voltage sampling point V2 at both ends of the pre-charge capacitor; the voltage sampling points and the data sampled by the DC Hall sensor are transmitted to the controller; Step Two: Fault Diagnosis During High-Voltage Power-On; specifically including the following steps: S21: The energy storage converter is in a shutdown state, and the controller sends a disconnect drive signal to relays K1 and K2; S22: Obtain the sampling values ​​of battery pack voltage sampling point V1, pre-charge capacitor voltage sampling point V2, and DC Hall sensor A1; if the value of DC Hall sensor A1 is greater than the preset charging current threshold value, it is determined that the main relay K2 is stuck and the controller issues a corresponding fault warning; otherwise, proceed to step S23. S23: Determine the magnitude of the sampled values ​​of the battery pack voltage sampling point V1 and the pre-charge capacitor voltage sampling point V2; if V2 is greater than a preset multiple of V1, it is determined that the pre-charge relay K1 is stuck at this time, and the controller issues a corresponding fault warning; otherwise, proceed to step S24. S24: Send a closing drive signal to relay K1; after waiting for a preset time, compare the sampled values ​​of battery pack voltage sampling point V1 and pre-charge capacitor voltage sampling point V2; if V2 is not greater than a preset multiple of V1, it is determined that the pre-charge relay K1 is open-circuited and the controller issues a corresponding fault warning; otherwise, proceed to step S25. S25: Send a closing drive signal to the main relay K2; after waiting for a preset time, obtain the sampling value of the DC Hall sensor A1; if A1 is not greater than the preset short-circuit current judgment threshold, it is determined that the main relay K2 is open-circuited and the controller issues a corresponding fault warning; otherwise, it is determined that the DC side pre-charging process is over. Step 3: Fault diagnosis during the operational phase: This includes the following steps: S31: The energy storage converter is in operation; determine whether the energy storage converter is in grid-connected mode; if yes, continue to step S32; if not, continue to step S33. S32: Obtain the sampling value of DC Hall sensor A1. If the sampling value of DC Hall sensor A1 is less than the preset no-load current, obtain the voltage fluctuation value of the voltage sampling point of the pre-charge capacitor. If the fluctuation value is greater than the preset DC fault voltage fluctuation value, it is determined that the main relay K2 is open at this time, and the controller issues a corresponding fault warning. S33: Obtain the sampling value of DC Hall sensor A1. If the sampling value of DC Hall sensor A1 is less than the preset no-load current, it is determined that the main relay K2 is open-circuited and the controller issues a corresponding fault warning.

[0007] Furthermore, the preset charging current threshold is 50A; the preset multiple is 0.95; the preset short-circuit current judgment threshold is 1A; the preset no-load current is 0.2A; and the preset DC fault voltage fluctuation value is 30V.

[0008] Furthermore, the preset time in step S25 is 2 seconds.

[0009] 3. Beneficial effects: (1) The present invention achieves diagnosis by using the original main relay, pre-charge relay, pre-charge resistor, battery pack voltage sampling circuit, DC capacitor voltage sampling circuit and DC side current sampling circuit of the energy storage converter. There is no need to add a voltage sampling circuit between the pre-charge relay and the negative DC bus, and a negative main relay is saved. The space saved can be optimized for structural design, and the cost advantage is also obvious.

[0010] (2) The relay design has a limited number of opening and closing cycles, which determines the relay's service life. Based on this, the present invention analyzes relay faults through the aforementioned voltage and current signals during the pre-charging process of the DC-side capacitor of the energy storage converter and during the normal operation state. This eliminates the need for multiple closing and opening of the relay to achieve fault diagnosis, which simplifies the diagnostic process and extends the relay's service life.

[0011] (3) Most current studies consider scenarios where the DC-side relay connects to the battery and the DC load, focusing only on the unidirectional flow of energy from the battery to the DC load and diagnosing relay faults during the brief transient process of power-on. This invention primarily targets the relay between the energy storage converter and the battery, where energy can flow bidirectionally, making fault diagnosis more complex. This invention can not only diagnose faults during the high-voltage power-on phase but also monitor the relay status online during operation, effectively ensuring the safe and reliable operation of the entire system. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the DC-side pre-charging circuit, controller, and circuit connection of the energy storage converter involved in this invention. Figure 2 This is a flowchart illustrating the fault diagnosis method for the high-voltage power-on stage of the energy storage converter used in this specific embodiment. Figure 3 This is a flowchart of the fault diagnosis method for the energy storage converter in the operating state stage, as used in a specific embodiment. Detailed Implementation

[0013] The present invention will now be described in detail with reference to the accompanying drawings.

[0014] Explanation of the principle of this solution; In order to solve the problems existing in the prior art, the present invention has taken the following measures: Firstly, based on the essential functional control and closed-loop pulse control of the energy storage converter, a DC relay, a pre-charging resistor, and sampling points for two voltage channels and one current channel are designed on the DC side. Specifically, the battery voltage sampling points are connected to the positive and negative buses of the battery, which are also the DC positive and negative input cables of the energy storage converter. The converter judges whether the external DC voltage input is normal by sampling the battery pack voltage, making it one of the essential sampling points for the energy storage converter. The DC capacitor voltage sampling points are connected to both ends of the DC-side capacitor of the energy storage converter. The capacitor plays the role of energy storage and filtering, so it is necessary to sample the voltage at this point for closed-loop control to realize the control output of the PWM wave and the constant voltage charging function, making it one of the essential sampling points for the energy storage converter. A DC current Hall sensor is placed on the DC positive main relay circuit. When the positive main relay is open, this circuit is not connected to the main circuit, so no current flows. When the positive main relay is closed, this circuit is connected to the main circuit, so charging current flows. Closed-loop control of the DC current achieves constant current charging of the battery voltage, and this is also one of the essential sampling points for the energy storage converter. The aforementioned positive main relay and current Hall sensor are connected in parallel across the pre-charge circuit formed by the pre-charge relay and pre-charge resistor. During the pre-charge phase, the energy storage converter closes the pre-charge relay, and the battery pack pre-charges the DC capacitor through the pre-charge resistor. Without a pre-charge circuit, the DC capacitor would experience a large instantaneous charging current due to the battery voltage connection, potentially damaging the capacitor. Furthermore, the pre-charge relay is only connected to the main circuit for a short period during the power-on phase. Because the pre-charge resistor limits the charging current, a relay with a smaller rated current can be selected, and thinner cables can be used for the connection wires, saving on component costs.

[0015] Secondly, based on the aforementioned hardware connections, a fault diagnosis scheme for the DC-side relays is proposed, specifically comprising the following steps: The battery voltage supply is normal, the energy storage converter has not received an operating command, and it is in standby mode. The DC positive main relay and the pre-charge relay have not received a closing drive signal; normally, they are in the open position, and the DC capacitor voltage is essentially zero. First, if the detected DC capacitor voltage value is close to the battery voltage, it indicates a sticking fault in either the DC positive main relay or the pre-charge relay. By detecting whether there is charging current through DC current, it can be determined which relay is sticking. If there is both DC current and charging current, it indicates that the current flows through the branch of the positive main relay, meaning the positive main relay is sticking. If no DC current flows, it indicates that the current flows through the branch of the pre-charge relay, meaning the pre-charge relay is sticking. Second, assuming the first step is normal, the energy storage converter sends a closing drive signal to the pre-charge relay. If the detected DC capacitor voltage value is close to the battery voltage, the pre-charge relay is closing normally; if after waiting for some time the DC capacitor voltage is still essentially zero, it indicates an open-circuit fault in the pre-charge relay. The third step, assuming the second step is successful, is to close the DC positive main relay. Due to the influence of the pre-charging resistor and line impedance, there is a certain voltage difference between the DC capacitor and the battery voltage. If the DC current detects a charging current, it indicates that the positive main relay is closed normally; if the DC current is essentially zero, it indicates that the positive main relay has an open circuit fault. At this point, the pre-charging relay is then disconnected, completing the detection steps for the energy storage converter during the initial high-voltage power-on phase on the DC side. When the energy storage converter is in off-grid operation, there is a certain power loss, the minimum of which is the no-load loss. Therefore, the DC current must have a minimum value. Thus, if the detected DC current value is less than the minimum no-load loss current, it is determined that the positive main relay is abnormally disconnected, indicating this fault. When the energy storage converter is charging or discharging in the grid, energy flows bidirectionally, and the DC side current will change from positive to negative or from negative to positive. When the minimum no-load current is detected, it needs to be combined with the DC side capacitor voltage value to make a judgment. This is because once the positive main relay is abnormally disconnected, the DC side capacitor voltage will run out of control due to the loss of battery voltage support. Therefore, if the DC current is detected at the minimum threshold and the DC side capacitor voltage fluctuation exceeds the normal threshold, it is judged that the DC side positive relay is abnormally disconnected. Specific implementation examples: The circuit connections involved in this embodiment are shown in the attached figure. Figure 1 As shown, in this embodiment, the specific connection methods of the battery pack voltage sampling point V1, DC capacitor voltage sampling point V2, DC Hall sensor A1, main relay K2, pre-charge relay K1, and pre-charge resistor R1 are as follows: Figure 1As shown. The allowable sampling error range for the two DC voltage values ​​is ±1V, and the sampling accuracy of the DC Hall sensor is less than 0.1%. The battery pack voltage V1 ranges from 1000V to 1500V. The DC capacitor is generally selected from 800uF to 4000uF, and the pre-charging resistor is generally selected from 100 to 500Ω. In the RC circuit, the charging time can be calculated using the RC time constant, τ = RC. Based on the exponential decay characteristic, the charging time can be approximated as 5 times the RC time constant, i.e., 5τ, with a charging time range between 0.4s and 10s. In this invention, the DC-side capacitor is selected as 1500uF, the pre-charging resistor as 200Ω, and the charging time is 1.2s.

[0017] The flowchart for the diagnostic method of the DC-side relay of the energy storage converter during the high-voltage power-on stage is attached. Figure 2 As shown in the diagram, the specific pre-charging circuit diagram is explained below. During the high-voltage power-on stage of the energy storage converter, the battery pack voltage V1 is normally supplied and is in a shutdown state. The main relay K2 and the pre-charging relay K1 do not receive a closing drive signal command. If the main relay K2 and the pre-charging relay K1 are not stuck together, they should be in the open position, the DC capacitor voltage V2 should be basically zero, and the DC current should also be basically zero. If the main relay K2 is stuck together, the battery pack voltage is equivalent to being instantaneously applied across the DC voltage terminals, resulting in a large current ripple. According to the DC capacitor charging formula: I=C*dV / dt, the current ripple will theoretically approach infinity. In actual circuits, it will be limited by factors such as circuit impedance, so the current ripple may not be infinitely large, but it will still be very high. Based on the above analysis, the charging current judgment threshold has a wide selection range, up to the rated current value of the DC Hall sensor of 300A. In this embodiment, the judgment charging current threshold is 50A. If the pre-charge relay K1 is stuck, the battery voltage V1 will charge the DC-side capacitor V2 through the pre-charge resistor R1. It is determined that the capacitor voltage V2 is approximately equal to the battery voltage V1. Considering the influence of battery characteristics and line impedance, the judgment threshold multiple used in this embodiment is V2 greater than 0.95V1. Therefore, when V2 is greater than 0.95V1, the pre-charge relay V1 has a sticking fault. If neither the main relay K2 nor the pre-charge relay K1 has a sticking fault, the converter sends a closing drive signal to the pre-charge relay K1. After waiting 2 seconds, if V2 is detected to be greater than 0.95V1, it indicates that the pre-charge relay K1 is closing normally; if V2 is not detected to be greater than 0.95V1 after waiting 2 seconds, it indicates that the pre-charge relay K1 has an open circuit fault. If the DC side voltage is basically fully charged, the positive main relay K2 is closed. At this time, it is equivalent to the branch containing K2 directly short-circuiting the branch containing K1. Due to the influence of pre-charge resistance and line impedance, there is still a certain voltage difference between the battery voltage and the DC capacitor voltage. Therefore, at the moment K2 closes, a short-circuit current will flow through the branch containing K2. This method uses a short-circuit current judgment threshold of 1A. If A1 is greater than 1A, then K2 is closed normally, and the pre-charging process on the DC side of the energy storage converter ends; otherwise, it indicates that K2 has an open circuit fault.

[0018] The flowchart of the diagnostic method for DC-side relays of energy storage converters during operation is as follows: Figure 3 As shown. Energy storage converters have two operating modes: grid-connected and off-grid. In grid-connected mode, they operate with current source characteristics, storing grid energy into the battery pack or feeding battery pack energy back to the grid. In off-grid mode, they operate with voltage source characteristics, providing power to the grid connected to the AC side of the energy storage system. This method can also diagnose online whether the main relay K2 has abnormally disconnected during the operation of the energy storage converter. First, it determines whether the energy storage converter is operating in off-grid or grid-connected mode. In off-grid mode, DC-side energy flows only unidirectionally, from the battery to the DC-side capacitor. When the energy storage converter is running under no-load, the DC-side current A1 has a minimum value. Considering that the no-load loss of the energy storage converter generally does not exceed 0.8% of its rated power, the no-load current for judging abnormal disconnection of K2 under off-grid conditions is set to 0.2A. When A1 is less than 0.2A, K2 is judged to have abnormally disconnected. When operating in grid-connected mode, energy flows bidirectionally on the DC side, and the DC current can be positive or negative. In this case, relying solely on the minimum value of A1 for diagnosis can easily lead to misjudgment. Another limiting condition needs to be added. When K2 is abnormally disconnected, according to the principle of energy conservation, the energy storage converter loses the energy source on one side, and the capacitor voltage will inevitably fluctuate abnormally. This method judges the abnormal fluctuation of the DC fault voltage to be ±30V. Based on these two conditions, the abnormal disconnection of the main relay K2 of the energy storage converter in grid-connected mode can be accurately diagnosed.

[0019] 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 method for diagnosing relay faults on the DC side of an energy storage converter, characterized in that: A DC-side pre-charge circuit is set on the DC side of the energy storage converter; the DC-side pre-charge circuit includes a battery pack and a pre-charge RC circuit; wherein the pre-charge capacitor C1 is connected in parallel with the DC side of the energy storage converter; one end of the pre-charge capacitor C1 is connected in series with the pre-charge resistor R1 and the pre-charge relay K1 and then connected to the positive terminal of the battery pack; the DC Hall sensor A1 is connected in series with the main relay K2 and then connected in parallel to the two ends of the series circuit formed by the pre-charge resistor R1 and the pre-charge relay K1. The other end of the pre-charge capacitor C1 is connected to the negative terminal of the battery pack; The fault diagnosis method includes the following steps: Step 1: Set sampling points; the sampling points include: setting battery pack voltage sampling point V1 at both ends of the battery pack, and setting pre-charge capacitor voltage sampling point V2 at both ends of the pre-charge capacitor; the voltage sampling points and the data sampled by the DC Hall sensor are transmitted to the controller; Step Two: Fault Diagnosis During High-Voltage Power-On; specifically including the following steps: S21: The energy storage converter is in a shutdown state, and the controller sends a disconnect drive signal to the pre-charge relay K1 and the main relay K2. S22: Obtain the sampling values ​​of battery pack voltage sampling point V1, pre-charge capacitor voltage sampling point V2, and DC Hall sensor A1; if the value of DC Hall sensor A1 is greater than the preset charging current threshold value, it is determined that the main relay K2 is stuck and the controller issues a corresponding fault warning; otherwise, proceed to step S23. S23: Determine the magnitude of the sampled values ​​of the battery pack voltage sampling point V1 and the pre-charge capacitor voltage sampling point V2; if V2 is greater than a preset multiple of V1, it is determined that the pre-charge relay K1 is stuck at this time, and the controller issues a corresponding fault warning; otherwise, proceed to step S24. S24: Send a closing drive signal to the pre-charge relay K1; after waiting for a preset time, compare the sampled values ​​of the battery pack voltage sampling point V1 and the pre-charge capacitor voltage sampling point V2; if V2 is not greater than a preset multiple of V1, it is determined that the pre-charge relay K1 is open-circuited and the controller issues a corresponding fault warning; otherwise, proceed to step S25. S25: Send a closing drive signal to the main relay K2; after waiting for a preset time, obtain the sampling value of the DC Hall sensor A1; if A1 is not greater than the preset short-circuit current judgment threshold, it is determined that the main relay K2 is open-circuited and the controller issues a corresponding fault warning; otherwise, it is determined that the DC side pre-charging process is over. Step 3: Fault diagnosis during the operational phase: This includes the following steps: S31: The energy storage converter is in operation; determine whether the energy storage converter is in grid-connected mode; if yes, continue to step S32; if not, continue to step S33. S32: Obtain the sampling value of DC Hall sensor A1. If the sampling value of DC Hall sensor A1 is less than the preset no-load current, obtain the voltage fluctuation value of the voltage sampling point of the pre-charge capacitor. If the fluctuation value is greater than the preset DC fault voltage fluctuation value, it is determined that the main relay K2 is open at this time, and the controller issues a corresponding fault warning. S33: Obtain the sampling value of DC Hall sensor A1. If the sampling value of DC Hall sensor A1 is less than the preset no-load current, it is determined that the main relay K2 is open-circuited and the controller issues a corresponding fault warning.

2. The method for judging DC-side relay faults in an energy storage converter according to claim 1, characterized in that: The preset charging current threshold is 50A; the preset multiple is 0.95; the preset short-circuit current judgment threshold is 1A; the preset no-load current is 0.2A; and the preset DC fault voltage fluctuation value is 30V.

3. The method for judging DC-side relay faults in an energy storage converter according to claim 1, characterized in that: The preset time in step S25 is 2 seconds.

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