Method for protecting energy storage converter, energy storage converter and energy storage system

Abstract

The present application relates to the field of energy storage, and discloses a protection method of an energy storage converter, the energy storage converter and an energy storage system.In the present application, the energy storage converter comprises a power conversion unit and a circuit breaker electrically connected to the direct current side of the power conversion unit, and the method comprises the following steps: obtaining a sampling current of the direct current side of the power conversion unit; obtaining a current rising rate of the direct current side from the sampling current; in the case that the current rising rate is greater than or equal to a first rising rate threshold, reducing the charge-discharge power of the energy storage converter; and according to the reduced charge-discharge power, reducing the overcurrent protection threshold of the circuit breaker.The present application can solve the problem that in the related art, when a short circuit fault occurs in the direct current side of the energy storage converter, the duration of the fault current flowing through the power conversion unit and the battery cluster is relatively long.

Description

Technical Field

[0001] This invention relates to the field of energy storage, and particularly to a protection method for an energy storage converter, an energy storage converter, and an energy storage system. Background Technology

[0002] An energy storage converter is an electrical device in an energy storage system that enables bidirectional conversion of electrical energy. An energy storage converter includes a power conversion unit; the DC side of the power conversion unit is connected to the battery cluster, and the AC side is connected to the power grid or load. The DC side of the energy storage converter is equipped with a circuit breaker, which has an overcurrent protection threshold and trips according to the overcurrent protection threshold to disconnect the DC circuit.

[0003] In related technologies, when a short-circuit fault occurs on the DC side of an energy storage converter, the fault current flows through the power conversion unit and the battery cluster for a relatively long period of time. Summary of the Invention

[0004] The purpose of this invention is to provide a protection method, energy storage converter, and energy storage system for an energy storage converter, in order to solve the problem in the related art that the fault current flows through the power conversion unit and the battery cluster for a long time when a short circuit fault occurs on the DC side of the energy storage converter.

[0005] To address the aforementioned technical problems, this invention provides a protection method for an energy storage converter. The energy storage converter includes a power conversion unit and a circuit breaker electrically connected to the DC side of the power conversion unit. The method includes: acquiring a sampled current on the DC side of the power conversion unit; obtaining the current rise rate on the DC side from the sampled current; reducing the charging and discharging power of the energy storage converter when the current rise rate is greater than or equal to a first rise rate threshold; and reducing the overcurrent protection threshold of the circuit breaker based on the reduced charging and discharging power.

[0006] The present invention also provides an energy storage converter for performing the above-described protection method for the energy storage converter.

[0007] The present invention also provides an energy storage system, including the above-described energy storage converter.

[0008] The present invention also provides an electrical device, including the above-mentioned energy storage converter.

[0009] The present invention also provides a control device, which is communicatively connected to an energy storage converter, and is used to execute the above-described protection method for the energy storage converter.

[0010] In this invention, by using the rate of current rise as the criterion for short-circuit faults, the rate of current rise increases as early as the current falls below the overcurrent protection threshold, allowing short-circuit faults to be identified in their early stages. The overcurrent protection threshold is limited by the normal operating current; as the charging / discharging power is reduced, the normal operating current decreases, and the lower bound of the overcurrent protection threshold shifts downward, thus allowing the overcurrent protection threshold to be lowered during the same fault development process. Furthermore, during charging, the reduction in charging / discharging power also decreases the current injected into the fault path by the power conversion unit. Therefore, the gap between the fault current and the overcurrent protection threshold is narrowed, the time required for the circuit breaker to meet operating conditions is shortened, and the duration of the fault current flowing through the power conversion unit and battery pack is reduced. Attached Figure Description

[0011] One or more embodiments are illustrated by way of example with reference numerals in the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0012] Figure 1 This is a schematic diagram showing the composition and connection relationship of an energy storage converter according to an embodiment of the present invention; Figure 2 A flowchart illustrating a protection method for an energy storage converter according to an embodiment of the present invention; Figure 3 A flowchart of adjusting the charging and discharging power is provided in one embodiment of the present invention; Figure 4 This is a flowchart of fault type determination and recovery provided in one embodiment of the present invention. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of 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 a part of the embodiments of the present invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0014] In this invention, "electrical connection" refers to a connection between two components that allows the transmission of electrical energy or signals. This includes direct connections via wires, busbars, or printed circuits, as well as indirect connections via circuit breakers, fuses, relays, or sampling elements. "Communicational connection" refers to a connection between two components that allows the transmission of signals, including via conductive media, optical media, or wireless methods. The difference lies in the fact that an electrical connection requires a current path between the two components, while a communication connection does not necessarily require a current path between the two components.

[0015] In this invention, "DC side" refers to the end of the power conversion unit connected to the battery cluster and its circuit, while "AC side" refers to the end of the power conversion unit connected to the power grid and its circuit. "Charging and discharging power" in this invention refers to the DC-side power exchanged by the power conversion unit under charging or discharging conditions, and its value is the product of the DC-side voltage and the DC-side current.

[0016] In this invention, the "current rise rate" refers to the rate of change of the DC-side current with respect to time, which can be obtained by differential or fitting of the sampled DC-side current; the "RMS value of the current" refers to the root mean square value of the DC-side current within a statistical time window, which can also be calculated from the sampled current. Both are physical quantities that can be directly calculated from the sampled current. The difference is that the former characterizes the rate of change of the current, while the latter characterizes the absolute level of the current.

[0017] In this invention, the terms "overcurrent protection threshold," "short circuit protection threshold," and "instantaneous protection threshold" all refer to the DC-side current setting values ​​corresponding to the circuit breaker's operating conditions. These values, from low to high, correspond to overload, short circuit, and extremely low impedance short circuit current levels, respectively. "Operating delay" refers to the adjustable waiting time from the moment the DC-side current reaches the corresponding protection threshold until the moment the circuit breaker issues a tripping command. The circuit breaker also requires an inherent delay from issuing the tripping command to completing the tripping operation. The sum of the operating delay and the inherent delay is the total operating delay. The terms "first," "second," and "third" in the names of the aforementioned thresholds, delays, and reduction values ​​are only used to distinguish different objects and are not used to indicate or imply relative importance.

[0018] In practical engineering deployments of energy storage systems, the energy storage converter undertakes bidirectional conversion between DC and AC power. Its DC side is connected to the battery cluster, and its AC side is connected to the power grid. The electrical protection of the DC side of the energy storage converter typically employs a three-stage protection scheme involving circuit breakers, fuses, and contactors. The circuit breaker trips according to a pre-set overcurrent protection threshold and tripping curve, balancing selectivity, economy, and safety. The circuit breaker's protection decision is derived from its own current detection mechanism; the circuit breaker only operates after the detected current reaches the overcurrent protection threshold.

[0019] However, the implicit premise of the above protection coordination is that the overcurrent protection threshold setting is higher than the DC side current during normal operation of the energy storage converter. When this premise is met, the circuit breaker will not malfunction under normal charging and discharging conditions, and can disconnect the fault circuit when the current significantly exceeds the normal range.

[0020] Analysis of this premise under actual operating conditions of the energy storage converter revealed that the energy storage converter has a wide operating power range, and its normal operating current on the DC side varies significantly with the charging and discharging power. To prevent the circuit breaker from malfunctioning under conditions such as full-power charging and discharging and inrush current during startup, the overcurrent protection threshold must be set according to the maximum possible normal operating current; that is, the higher the operating power, the higher the overcurrent protection threshold setting.

[0021] The DC-side short-circuit current does not have a natural zero-crossing point, and its rate of rise is much higher than that of the AC-side short-circuit current. After a short circuit occurs, the DC-side current rises from the normal operating current level and must climb above the overcurrent protection threshold before the circuit breaker is ready to operate. The higher the overcurrent protection threshold setting, the longer it takes for the current to rise from the moment of the fault to that threshold, and during this time, the fault current continues to flow through the power conversion unit and the battery pack.

[0022] The deeper mechanism lies in the fact that changing the overcurrent protection threshold to be dynamically adjusted based on the operating current under different operating conditions only eliminates the one-size-fits-all threshold across all operating conditions; the constraint that "the threshold must be higher than the current normal operating current" remains unchanged. Under high-power operating conditions, the overcurrent protection threshold obtained by dynamic adjustment remains high, and therefore the lower limit of the time required for the fault current to climb to that threshold remains unchanged. At the same time, the battery cluster has low DC internal resistance, and the amplitude of the DC side short-circuit current can reach tens of times the rated current. The longer the fault current flows through the power conversion unit and the battery cluster, the more fault energy is injected into the battery cluster, and the risk of internal temperature rise and thermal runaway propagation accumulates over this time.

[0023] The essence of the aforementioned problem is not whether the overcurrent protection threshold changes with operating conditions, but rather that the overcurrent protection threshold setting is always based on the current normal operating current as the lower bound, while the normal operating current is fixed at the moment of fault occurrence. Related technical solutions all focus on the overcurrent protection threshold, either improving the threshold setting accuracy or shortening the threshold refresh cycle. However, the normal operating current itself is not affected during the fault development process, thus failing to lower the achievable lower bound of the overcurrent protection threshold.

[0024] This invention identifies the deficiency in intervening in the normal operating current itself during the fault development process. It proposes a method: using the current rise rate on the DC side of the power conversion unit as an entry point, short-circuit precursors are identified while the DC side current is still below the overcurrent protection threshold. Upon identification of the short-circuit precursor, the charging and discharging power of the energy storage converter is proactively reduced, lowering the normal operating current on the DC side. Then, the overcurrent protection threshold is lowered in tandem with the reduced charging and discharging power. This scheme replaces the passive following of the overcurrent protection threshold with proactive intervention in the operating power, narrowing the gap between the fault current and the protection threshold simultaneously without altering the circuit breaker's breaking capacity or tripping mechanism.

[0025] The protection method, energy storage converter, energy storage system, electrical equipment and control equipment provided in the embodiments of the present invention can be applied to DC side electrical protection scenarios of power conversion equipment such as energy storage converters, photovoltaic inverters, and frequency converters.

[0026] The following is a detailed description of the implementation details of the protection method for the energy storage converter according to an embodiment of the present invention. The following content is only for the convenience of understanding and is not necessary for implementing this solution.

[0027] Reference Figure 1 , Figure 1 This is a schematic diagram illustrating the composition and connection relationship of an energy storage converter according to an embodiment of the present invention. The energy storage converter includes a power conversion unit and a circuit breaker, with the circuit breaker electrically connected to the DC side of the power conversion unit. The DC side of the power conversion unit is electrically connected to the battery cluster via the circuit breaker, and the AC side of the power conversion unit is electrically connected to the power grid.

[0028] The power conversion unit includes a conversion bridge arm composed of multiple power devices, as well as a filter inductor and a filter capacitor. The power devices can be insulated gate bipolar transistors or silicon carbide metal oxide semiconductor field-effect transistors. In the charging condition, it converts the AC power on the AC side into DC power to charge the battery pack, and in the discharging condition, it converts the DC power from the battery pack into AC power to output to the AC side.

[0029] like Figure 1As shown, the energy storage converter also includes a converter control unit and a protection setting unit. The converter control unit is electrically connected to the power conversion unit and is used to collect the operating electrical quantities of the power conversion unit and issue power commands to the power conversion unit. The protection setting unit is electrically connected to the circuit breaker and is used to write overcurrent protection thresholds and operating delays to the circuit breaker. The converter control unit and the protection setting unit are communicatively connected via the internal signal channel of the energy storage converter. Integrating the protection setting unit, circuit breaker, and power conversion unit into the same energy storage converter and directly connecting them via the internal signal channel ensures that the power commands on which the threshold setting is based and the execution of the power commands are within the same control domain, and that there is no communication link between them outside the energy storage converter.

[0030] Reference Figure 2 , Figure 2 A flowchart illustrating a protection method for an energy storage converter according to an embodiment of the present invention. See also... Figure 2 The method includes steps S100 to S400.

[0031] Step S100: Obtain the sampling current on the DC side of the power conversion unit.

[0032] The sampling current refers to the sequence of current values ​​obtained by sampling the current flowing through the DC-side circuit of the power conversion unit. In one example, the sampling current is obtained by a shunt, Hall current sensor, or Rogowski coil located on the DC-side circuit, with the sampling point between the circuit breaker and the power conversion unit. The shunt reflects the current through the voltage drop across the sampling resistor, the Hall current sensor reflects the current through the magnetic field effect, and the Rogowski coil, fitted outside the DC-side conductor, reflects the current change through induced electromotive force.

[0033] In another example, the sampled current is obtained by sampling from the drive circuit of the power devices in the power conversion unit. The sampled values ​​from each power device are synthesized according to the switching state of the power conversion unit, without the need for additional sampling elements in the DC-side circuit. This method is suitable for situations where the current in a fault path flows through the power devices; when the fault path is located between the circuit breaker and the power conversion unit, the DC-side circuit sampling in the previous example is used. This invention does not limit the specific method of obtaining the sampled current.

[0034] It should be noted that the sampling frequency of the sampled current in this invention is greater than the current sampling frequency of the circuit breaker. In a specific example, the sampling frequency of the sampled current can be from 10 kHz (kilohertz) to 1 MHz (megahertz), for example, 10 kHz, 20 kHz, 50 kHz, 100 kHz, 500 kHz, or 1 MHz; the current sampling frequency of the circuit breaker is typically from 1 kHz to 5 kHz, which matches the millisecond-level mechanical action time of the circuit breaker's tripping mechanism. The current rise rate is a quantity obtained from the difference in current between adjacent sampling moments. The shorter the sampling interval, the more completely the current rise process in the short-circuit precursor stage is preserved in the sampling results. The current rise rate obtained is closer to the actual rate of change of this process, and the moment when the current rise rate reaches the first rise rate threshold is also closer to the moment when it actually rises.

[0035] This feature also ensures the effectiveness of the closed loop formed by steps S300 and S400. This closed loop needs to complete three stages—short-circuit precursor identification, charging / discharging power reduction, and overcurrent protection threshold reduction—before the DC-side current rises to the overcurrent protection threshold before it is lowered. Its effectiveness depends on the advance of short-circuit precursor identification relative to the fault occurrence. A sampling frequency higher than the circuit breaker's current sampling frequency allows short-circuit precursors to be identified before the circuit breaker's own detection circuitry obtains sufficient sampling results to reflect the current rise. Therefore, the above three stages start earlier, and the charging / discharging power and overcurrent protection threshold reduction are completed when the fault current is at a lower level. Consequently, the duration of the fault current flowing through the power conversion unit and battery pack is further shortened.

[0036] The sum of the times occupied by the above three steps should be shorter than the time required for the DC-side current to climb from the moment of the fault to the overcurrent protection threshold before the reduction. In a specific example, for an energy storage converter with a rated power of 1MW and a DC-side voltage of 1000V, its rated DC-side current is 1000A, and the overcurrent protection threshold is set at twice the rated current (2000A). When the current rise rate is 1kA / ms, it takes 1ms for the DC-side current to climb from the rated operating level to this threshold. Accordingly, the current rise rate is calculated using a sliding time window of 0.05ms to 0.2ms, for example, 0.05ms, 0.1ms, 0.15ms, or 0.2ms. The reduction of the current limit value takes effect within one switching cycle through the peak current comparison circuit, and the overcurrent protection threshold is written through a hard-wired trigger channel or a high-speed serial bus. The sum of the three times is within the above-mentioned time. When the current rise rate is higher, the reduction in charging and discharging power should be increased accordingly according to the tiered reduction below.

[0037] The specific value of the sampling frequency mentioned above should satisfy the operation cycle capability of the control chip in the power conversion unit.

[0038] Step S200: Obtain the current rise rate on the DC side from the sampled current.

[0039] The common unit for rate of rise of current is A / ms (amperes per millisecond) or kA / ms (kiloamperes per millisecond). In one example, the rate of rise of current is obtained by dividing the difference between the sampled currents at adjacent sampling times by the sampling period; in another example, the rate of rise of current is obtained by least-squares fitting of multiple sampled currents within a sliding time window to obtain the slope of the current change within that time window. The duration of the sliding time window can be from 0.05ms to 1ms, for example, 0.05ms, 0.1ms, 0.2ms, 0.5ms, or 1ms. Using a sliding time window fitting method helps to suppress the influence of single-point sampling noise on the calculation result of the rate of rise of current. The duration of the sliding time window constitutes the calculation delay of the rate of rise of current. Under the condition that the fault path impedance changes rapidly, the lower limit of the above range is taken so that the delay is shorter than the time required for the DC side current to climb to the overcurrent protection threshold.

[0040] Step S300: If the current rise rate is greater than or equal to the first rise rate threshold, reduce the charging and discharging power of the energy storage converter.

[0041] The first rise rate threshold is a threshold for the current rise rate used to determine short-circuit precursors. In a specific example, under normal charging and discharging conditions, the DC-side current rise rate of an energy storage converter is in the range of several hundred A / ms; when short-circuit precursors appear on the DC side, the current rise rate rises to 1kA / ms to 5kA / ms; after the short circuit enters its initial stage, the current rise rate can reach over 10kA / ms. Accordingly, the first rise rate threshold can be set to 1kA / ms to 5kA / ms, for example, 1kA / ms, 2kA / ms, 3kA / ms, 4kA / ms, or 5kA / ms. This threshold value should be higher than the current rise rate caused by power command regulation and load fluctuations under normal charging and discharging conditions, while being lower than the current rise rate in the short-circuit precursor stage, thus meeting the above distinction requirements.

[0042] The physical basis for using the rate of current rise as a criterion is as follows: the rate of current rise on the DC side is equal to the ratio of the net voltage across the inductor of the DC-side circuit to the inductance of that circuit, and this net voltage is equal to the electromotive force (EMF) of the battery cluster minus the voltage drop across the fault path. The lower the impedance of the fault path, the smaller the voltage drop across the fault path, the closer the net voltage is to the EMF of the battery cluster, and the higher the rate of current rise. During normal operation, the voltage drop across the current path is maintained by the control of the energy storage converter, the net voltage is very small, and the rate of current rise is correspondingly low. The rate of current rise thus becomes a measure of the impedance of the fault path: when the fault path is in a high-impedance stage, the net voltage is only a small portion of the EMF of the battery cluster, and the rate of current rise is at a moderate level; after the fault path develops into a metallic contact, the net voltage approaches the EMF of the battery cluster, and the rate of current rise increases significantly. The change in the impedance of the fault path is reflected in the rate of current rise at the same time it forms, while the current amplitude needs a period of accumulation to reach the overcurrent protection threshold. Therefore, the change in the rate of current rise crosses the protection threshold earlier than the current amplitude.

[0043] In a specific example, the inductance of the DC-side circuit is 100 μH (microhenries), and the electromotive force of the battery cluster is 1500 V: when the voltage drop across the fault path is 1400 V, the net voltage is 100 V, and the current rise rate is 1 kA / ms; when the voltage drop across the fault path is 500 V, the net voltage is 1000 V, and the current rise rate is 10 kA / ms. It can be seen that the difference in the current rise rate between the precursor stage and the initial stage of a short circuit corresponds to the difference in the order of magnitude of the fault path impedance.

[0044] This invention intervenes when the fault path is still at a high impedance stage. At this stage, the DC side voltage has experienced a measurable drop but has not yet collapsed, and the normal operating current still flows between the battery pack and the power conversion unit through the circuit breaker. Therefore, it is still controlled by the current limiting link of the power conversion unit, and the charging and discharging power is reduced so that the current measured by the circuit breaker is reduced.

[0045] When the DC side voltage collapses, the charging and discharging power and the overcurrent protection threshold both tend to their lower limit, and the circuit breaker then becomes capable of operating; the electrical isolation of the fault path is accomplished by the blocking power conversion unit and the control circuit breaker tripping as described below.

[0046] Lowering the charging and discharging power of the energy storage converter means reducing the actual DC-side power exchanged by the power conversion unit to a level lower than before the fault occurred.

[0047] In one example, the reduction in charging and discharging power is achieved by lowering the DC-side current limit of the power conversion unit. The current limit can be the quotient of the reduced charging and discharging power and the DC-side voltage. This current limiting is executed by the peak current comparison circuit of the power conversion unit. When the DC-side current reaches the current limit, the comparison circuit cuts off the drive pulse of the power device within the current switching cycle. Therefore, the reduction in the current limit takes effect within one switching cycle, which lasts from tens to hundreds of microseconds. After the DC-side current is limited, under the condition of a fixed DC-side voltage, the actual DC-side power exchanged by the power conversion unit decreases accordingly. The peak current comparison circuit and the sampling of the power device's drive circuit in step S100 are in the same loop. The sampling and execution do not pass through the outer power loop, and their time scales are matched.

[0048] In another example, reducing the charging and discharging power is achieved by issuing a reduced active power command to the power conversion unit. This method is executed via the power outer loop, and its effective time depends on the power conversion unit's power change rate capability, falling on the order of milliseconds. It is suitable for operating conditions where the fault path impedance changes slowly. The above two methods can be implemented individually or in combination.

[0049] Reference Figure 3 , Figure 3 This is a flowchart illustrating the reduction of charge and discharge power according to one embodiment of the present invention. Optionally, as shown... Figure 3 As shown, the method further includes: determining the effective value of the DC current based on the sampled current; and reducing the charging and discharging power of the energy storage converter when the current rise rate is greater than or equal to a first rise rate threshold, including: reducing the charging and discharging power of the energy storage converter when the current rise rate is greater than or equal to the first rise rate threshold and the effective value of the current is greater than or equal to a first current threshold. In a specific example, the effective value of the current is calculated using a statistical time window of 0.1ms to 2ms, for example, 0.1ms, 0.5ms, 1ms, or 2ms; the duration of the statistical time window constitutes the calculation delay of the criterion, and the lower limit of the above range is taken under the condition that the fault path impedance changes rapidly, so that the delay is within the time allowed by the aforementioned three steps. The first current threshold should be lower than the overcurrent protection threshold before the adjustment, so that the criterion is established before the current amplitude exceeds the threshold. When the overcurrent protection threshold is set to twice the rated current, the first current threshold can be 1.2 to 1.8 times the rated current, for example, 1.2, 1.4, 1.6, or 1.8 times the rated current. Using both as criteria simultaneously helps to distinguish between short-circuit precursors and operating conditions such as power command step jumps and inrush currents during startup, where the current rise rate increases temporarily but the current level remains within the normal range.

[0050] Optionally, such as Figure 3As shown, step S300, which involves reducing the charging and discharging power of the energy storage converter, includes: reducing the charging and discharging power by a first reduction when the current rise rate is greater than or equal to a third rise rate threshold; and reducing the charging and discharging power by a second reduction when the current rise rate is greater than or equal to the first rise rate threshold and less than the third rise rate threshold; wherein the third rise rate threshold is greater than the first rise rate threshold and the first reduction is greater than the second reduction. In a specific example, the third rise rate threshold is greater than the first rise rate threshold and less than the second rise rate threshold. It can be 1.1 to 1.8 times the first rise rate threshold. For example, when the first rise rate threshold is 3kA / ms and the second rise rate threshold is 6kA / ms, the third rise rate threshold can be 3.3kA / ms, 4kA / ms, 4.5kA / ms, or 5kA / ms. The first descent can be 50% to 100% of the rated power, for example, 50%, 70%, 80%, or 100%. The second descent can be 10% to 50% of the rated power, for example, 10%, 20%, 30%, or 50%. Figure 3 This example illustrates the combination of the current RMS criterion and the tiered reduction, although both can also be implemented separately.

[0051] A higher current rise rate indicates a lower impedance in the fault path, faster fault development, and shorter time for the protection circuit. Adjusting the charging and discharging power by a larger reduction margin helps to align the energy reduction with the speed of fault development. Determining the reduction margin by grade also ensures that the amount of advance the circuit breaker meets the operating conditions tends to be consistent across different fault development speeds: this advance margin is the quotient of the narrowing amount (described later) and the current rise rate. An increase in the current rise rate decreases this quotient, while the reduction margin increases in the same direction as the current rise rate, thus offsetting each other. The specific reduction margin for each grade should meet the power conversion unit's power change rate capability.

[0052] Optionally, the method further includes: when the current rise rate is greater than or equal to a first rise rate threshold, obtaining the voltage on the DC side of the power conversion unit and the harmonic distortion rate on the AC side of the power conversion unit; weighting the current rise rate, the voltage on the DC side, and the harmonic distortion rate according to a set weight to obtain an action confidence level; the action confidence level characterizes the degree of confidence in the occurrence of a short-circuit fault; when the action confidence level is greater than or equal to a confidence threshold, reducing the charging and discharging power.

[0053] In a specific example, the current rise rate, DC-side voltage, and harmonic distortion rate are each normalized to dimensionless membership degrees based on their respective normal operating ranges before weighting. The higher the current rise rate, the deeper the DC-side voltage drop, and the lower the harmonic distortion rate, the closer the membership degree is to 1. The sum of the weights of the three is 1. The weight of the current rise rate can be 0.3 to 0.5, for example, 0.3, 0.4, or 0.5; the weight of the DC-side voltage can be 0.3 to 0.5, for example, 0.3, 0.4, or 0.5; and the weight of the harmonic distortion rate can be 0.1 to 0.3, for example, 0.1, 0.2, or 0.3. The confidence threshold can be 0.6 to 0.9, for example, 0.6, 0.7, 0.8, or 0.9. When a short circuit occurs on the DC side, the increase in current rise rate and the drop in DC side voltage occur simultaneously. In contrast, current fluctuations caused by AC side disturbances are typically accompanied by an increase in harmonic distortion rate, while the DC side voltage drop is limited. Therefore, both factors share the primary role in the discrimination. This weighted calculation is completed within the same control cycle when the current rise rate reaches the first rise rate threshold, and its time delay is within the allowable time of the aforementioned three stages. Thus, multi-parameter weighting is beneficial for maintaining charging and discharging power under transient conditions such as lightning strikes, switching overvoltages, and commutation fluctuations. This invention does not limit the specific values ​​of the weights and confidence thresholds.

[0054] Step S400: Adjust the overcurrent protection threshold of the circuit breaker according to the reduced charging and discharging power.

[0055] The basis for lowering the overcurrent protection threshold with increasing charging and discharging power is that the lower limit of the overcurrent protection threshold is determined by the normal operating current on the DC side, which in turn is determined by the ratio of the charging / discharging power to the DC side voltage. After lowering the charging / discharging power in step S300, the normal operating current on the DC side decreases, and the lower limit of the overcurrent protection threshold shifts accordingly. Therefore, the overcurrent protection threshold can be lowered to above the new normal operating current without causing malfunction. In a specific example, the overcurrent protection threshold can be determined according to the relationship between the reference threshold and the power deviation, as shown in the formula I_new=I_base×(1+k×ΔP / P_rated), where I_new is the reduced overcurrent protection threshold in A (amperes); I_base is the reference threshold, i.e., the overcurrent protection threshold under rated power conditions, in A; k is the load sensitivity coefficient, a dimensionless quantity, and the larger the k, the stronger the overcurrent protection threshold's response to power deviation; ΔP is the deviation of the reduced charging and discharging power from the rated power, in kW (kilowatts), and ΔP becomes negative after the charging and discharging power is reduced; P_rated is the rated power of the energy storage converter, in kW.

[0056] The load sensitivity coefficient must be chosen such that the decrease in the overcurrent protection threshold is greater than the decrease in the normal operating current on the DC side. This is a condition that shortens the time required for the circuit breaker to meet the operating conditions. Let the overcurrent protection threshold under rated power conditions be n times the rated current on the DC side. Then, this condition is that the load sensitivity coefficient is greater than the reciprocal of n: when the load sensitivity coefficient equals the reciprocal of n, the decrease in the overcurrent protection threshold is equal to the decrease in the normal operating current, and the difference between the two remains constant; when the load sensitivity coefficient is less than the reciprocal of n, this difference widens. When the load sensitivity coefficient is 1, the overcurrent protection threshold is proportional to the charging and discharging power, and the decrease in the threshold is n times the decrease in the normal operating current, resulting in the greatest narrowing of the difference. Furthermore, this value satisfies the above condition for any n greater than 1, and is therefore the preferred value. In a specific example, when n is 2, the load sensitivity coefficient can be a value greater than 0.5 and not greater than 1, such as 0.6, 0.7, 0.8, 0.9 or 1.0; when n is 1.25, the load sensitivity coefficient can be a value greater than 0.8 and not greater than 1, such as 0.85, 0.9, 0.95 or 1.0.

[0057] The load sensitivity coefficient is set to no greater than 1, ensuring that the overcurrent protection threshold remains above zero even under extreme operating conditions where the charging / discharging power is reduced to zero. Furthermore, the reduced overcurrent protection threshold is not lower than a set multiple of the reduced DC-side normal operating current, which can be between 1.2 and 2, for example, 1.2, 1.5, or 2. This lower limit is also not lower than the circuit breaker's minimum adjustable value, which can be between 0.05 and 0.2 times the DC-side rated current, for example, 0.05, 0.1, or 0.2 times. Thus, the overcurrent protection threshold maintains a margin relative to the current normal operating current while adjusting with the charging / discharging power, and maintains a settable current level even when the charging / discharging power is reduced to zero. The short-circuit protection threshold and the instantaneous protection threshold, when determined as a multiple of the overcurrent protection threshold, also maintain the same margin relationship. The lower bound moves down synchronously with the reduced normal operating current, so that the overcurrent protection threshold keeps following the power reduction range throughout the entire charging and discharging power reduction range. If the lower bound is a fixed value that is independent of the charging and discharging power, the overcurrent protection threshold will be limited by it and will no longer follow the power reduction under the condition of large reduction. Its reduction amount is less than the reduction amount of the normal operating current, and the aforementioned gap cannot be narrowed.

[0058] In one example, the overcurrent protection threshold is set to the normal operating current corresponding to the quotient of the adjusted charge / discharge power and the current DC-side voltage. The DC-side voltage of the energy storage converter changes with the state of charge of the battery pack and droops during faults. The overcurrent protection threshold is adjusted to match the actual normal operating current based on the current DC-side voltage.

[0059] As a result, the overcurrent protection threshold is lowered during the same fault development process. The time required for the DC side current to climb from the lowered normal operating current level to the lowered overcurrent protection threshold is shorter than the time required to climb to the original overcurrent protection threshold, and the circuit breaker is ready to operate earlier.

[0060] The degree of time reduction mentioned above can be expressed quantitatively: the reduced time is equal to the quotient of the narrowing of the gap between the fault current and the overcurrent protection threshold and the rate of increase of the fault current. This narrowing is equal to the decrease in the overcurrent protection threshold minus the decrease in the current measured by the circuit breaker. The overcurrent protection threshold is determined by the adjusted charging and discharging power, and its decrease is determined by the power command. The decrease in the current measured by the circuit breaker is reduced according to the current shunting relationship between the fault path impedance and the battery cluster branch impedance; the lower the impedance of the fault path, the smaller the decrease. Therefore, as the fault path impedance decreases, the former remains unchanged while the latter decreases, and the narrowing increases accordingly. The aforementioned time reduction holds true at all stages of the fault path impedance reduction.

[0061] Under discharge conditions, the current in the fault path is mainly fed in by the battery cluster, and its rate of rise depends on the electromotive force of the battery cluster and the inductance of the DC-side circuit, and does not change with the charging and discharging power. Under charging conditions, the current measured by the circuit breaker is the charging current before the fault occurs, and turns into the current fed out by the battery cluster to the fault path during the fault development. The amount of current change required for it to increase from the charging current level to the overcurrent protection threshold includes the charging current itself, and the narrowing is therefore greater than under discharge conditions. In addition, the power conversion unit is also a source of fault path current under charging conditions. The reduction in charging and discharging power reduces the current injected into the fault path through the controllable path, and the amplitude and rate of rise of the fault path current are reduced accordingly.

[0062] It should be noted that the two benefits mentioned above apply to different currents. One benefit is achieved by shortening the action time, while the other is achieved by reducing the current level. When the fault path is located between the circuit breaker and the power conversion unit, the current measured by the circuit breaker is the current fed from the battery pack to the fault path through the circuit breaker. Its rate of rise does not decrease with the reduction of charging and discharging power, and the moment when the circuit breaker meets the operating conditions is thus advanced according to the above relationship. The reduction in the current injected into the fault path by the power conversion unit through its controllable path is reflected in the reduction of the total current amplitude of the fault path. When the fault path is located between the battery pack and the circuit breaker, the current injected by the power conversion unit flows through the circuit breaker. This reduction in current reduces the total current of the fault path and also reduces the rate of rise of the current measured by the circuit breaker. In this case, the reduction of charging and discharging power mainly reduces the level of the fault current.

[0063] It should be noted that after the overcurrent protection threshold is lowered, the absolute difference between it and the normal operating current on the DC side decreases accordingly. To ensure that this decrease in difference is compatible with the protection's disturbance rejection capability, the determination of short-circuit precursors can be constrained according to the aforementioned current effective value criterion and action confidence determination, so that the reduction of charging and discharging power and the reduction of the overcurrent protection threshold are executed under the condition that short-circuit precursors are established.

[0064] Steps S300 and S400 form a closed loop in terms of timing: step S300 changes the actual power exchanged by the power conversion unit, and step S400 changes the protection threshold of the circuit breaker. Both act on both sides of the fault current and the protection threshold, and their common result is that the gap between the two is narrowed simultaneously.

[0065] It should be noted that the scenario where the current rise rate is greater than or equal to the first rise rate threshold and the scenario where the current rise rate is greater than or equal to the second rise rate threshold may occur simultaneously. When both scenarios occur simultaneously, the scenario with the highest current rise rate will be processed, that is, the action corresponding to the second rise rate threshold will be executed first. The reason for this is that the fault corresponding to the second rise rate threshold is more advanced, and its response measures cover the response measures corresponding to the first rise rate threshold in terms of energy cut-off force.

[0066] In some embodiments, the method for determining the overcurrent protection threshold in step S400 is further explained below.

[0067] The essential function of step S400 is to determine an overcurrent protection threshold value that is appropriate for the current operating state. This value should be higher than the current normal operating current but lower than the current withstand capability of the protected object under this operating state. The current withstand capability of the protected object can be characterized in various ways, such as the maximum permissible instantaneous current of the battery pack, the short-time thermal withstand current of the DC cable, and the surge current withstand capability of the power device. This embodiment uses the maximum permissible instantaneous current of the battery pack as an example for explanation.

[0068] In some embodiments, the circuit breaker is electrically connected to the battery cluster; the method further includes: acquiring the state of charge and cell temperature of the battery cluster; determining the maximum permissible instantaneous current of the battery cluster based on the state of charge and cell temperature; determining a battery protection threshold based on the maximum permissible instantaneous current and charge / discharge power; wherein, in adjusting the overcurrent protection threshold of the circuit breaker based on the reduced charge / discharge power, the reduced overcurrent protection threshold of the circuit breaker is not greater than the battery protection threshold.

[0069] In a specific example, the state of charge (SOC) and cell temperature of the battery cluster are collected and output by the battery cluster management unit. The SOC ranges from 0% to 100%, for example, 10%, 30%, 50%, 80%, or 100%; the cell temperature ranges from -20°C to 60°C, for example, -20°C, 0°C, 25°C, 45°C, or 60°C. The DC internal resistance of the battery cluster changes with the SOC and cell temperature: at low cell temperatures and low SOCs, the internal resistance increases, and the heat generation and polarization voltage inside the cell increase at the same current, resulting in a decrease in the instantaneous current it can withstand; at high cell temperatures and medium SOCs, the internal resistance decreases, and the instantaneous current it can withstand increases; the high cell temperature range is also constrained by the cell's thermal safety boundary, so the maximum permissible instantaneous current does not change monotonically with cell temperature. Therefore, the maximum permissible instantaneous current is obtained by looking up a table or interpolating from the SOC and cell temperature.

[0070] In a specific example, the battery protection threshold is determined jointly by the maximum permissible instantaneous current and the charge / discharge power: the maximum permissible instantaneous current gives the upper limit of the battery pack's current in the current state, and the ratio of the charge / discharge power to the DC-side voltage gives the current operating current level on the DC side. The battery protection threshold is taken between this operating current and the maximum permissible instantaneous current, which is higher than the current operating current to avoid false tripping, but does not exceed the current carrying capacity of the battery pack. The reduced overcurrent protection threshold is limited to no more than the battery protection threshold, so that the overcurrent protection threshold, while decreasing with the charge / discharge power, is also constrained by the capped current carrying capacity of the battery pack.

[0071] By using the maximum permissible instantaneous current of the battery cluster as the overcurrent protection threshold, the value of the overcurrent protection threshold is constrained by both the lower limit of the operating current and the upper limit of the battery cluster's withstand capacity. Compared to determining the overcurrent protection threshold solely based on the operating current, this approach helps to prevent the fault current from reaching a level that the battery cluster cannot withstand before the circuit breaker trips, especially under conditions where the battery cluster's withstand capacity decreases, such as low cell temperature or low state of charge.

[0072] It should be noted that under operating conditions where the battery protection threshold is inherently low, such as low cell temperature or low state of charge, the battery protection threshold may be lower than the lower limit determined by the adjusted normal operating current. Under such conditions, the overcurrent protection threshold is taken as the capped constraint of the battery protection threshold, and the reduction of fault energy is directly achieved by reducing the charging and discharging power.

[0073] In some embodiments, the handling method when the current rise rate further increases is further described below.

[0074] Steps S300 and S400 reduce fault energy during the short-circuit precursor stage. Their function is essentially to reduce the fault current level while maintaining grid-connected operation of the energy storage converter. When the current rise rate continues to increase to indicate that the short circuit has entered its initial stage, the impedance of the fault path has dropped to an even lower level. At this point, it is necessary to simultaneously disconnect the fault at both the energy source and electrical path levels. This disconnection process can be achieved in various ways, such as blocking the power conversion unit, controlling the circuit breaker to trip, or disconnecting the relay unit. This embodiment uses a combination of blocking the power conversion unit and controlling the circuit breaker to trip as an example.

[0075] In some embodiments, the method further includes: blocking the power conversion unit when the current rise rate is greater than or equal to a second rise rate threshold; the second rise rate threshold is greater than a first rise rate threshold; and controlling the circuit breaker to perform tripping.

[0076] In a specific example, the second rise rate threshold can be between 6kA / ms and 15kA / ms, for example, 6kA / ms, 8kA / ms, 10kA / ms, or 15kA / ms, which is higher than the first rise rate threshold. After the fault path develops into metallic contact, the current rise rate can further reach over 100kA / ms. Blocking the power conversion unit refers to blocking the drive signal of the power device, causing all power devices to turn off. This action is directly executed by the drive circuit, and its completion time is in the microsecond range. Controlling the circuit breaker to perform opening refers to separating the contacts of the circuit breaker and completing the arc extinguishing. This action involves the mechanism action and the arc extinguishing process, and its completion time is in the millisecond range. In one example, blocking the power conversion unit and controlling the circuit breaker to perform opening are executed sequentially. First, the power conversion unit is blocked, preventing it from injecting energy into the fault path through its controllable path. Subsequently, the circuit breaker completes the electrical isolation of the DC side circuit. It should be noted that all power devices are connected in anti-parallel with diodes. After the power conversion unit is blocked, the AC side can still feed current to the DC side through the diode path, and the battery pack can still feed current to the fault path through the circuit breaker. The electrical isolation of the fault path is completed by the circuit breaker opening.

[0077] Therefore, in this embodiment, when the fault current rise rate indicates that the short circuit has entered the initial stage, the power conversion unit is first blocked by a microsecond-level drive signal to stop the energy injection through the controllable path, and then the electrical isolation is completed by a millisecond-level mechanism action. Compared with the case where the circuit breaker mechanism action is used to complete the disconnection, the energy injection of the controllable path can be stopped earlier than the completion of the electrical isolation, which is beneficial to limit the total amount of energy injected into the fault path during the period before the circuit breaker arc extinguishing process has ended.

[0078] In some embodiments, the distinction between fault characteristics and power restoration methods after a short circuit enters its initial stage is further explained below.

[0079] DC-side short-circuit faults can be classified into two categories based on the maintenance conditions of their fault paths: transient faults and permanent faults. Transient faults are formed by transient excitations such as lightning strikes or switching overvoltages, and the path ceases to exist after the excitation disappears. Permanent faults are formed by metallic contacts or carbonization of the insulating medium, and persist even after the excitation disappears. Applying the same treatment to both types of faults would cause unnecessary and prolonged power outages due to transient faults. Distinguishing the nature of the fault path can be achieved in various ways, such as extracting the oscillation characteristics of voltage and current, injecting a test signal to measure the equivalent impedance of the circuit, and comparing the insulation resistance before and after the fault. This embodiment uses the extraction of the oscillation characteristics of DC-side voltage and current as an example for illustration.

[0080] Optionally, the fault nature differentiation step in this embodiment is set within the aforementioned disconnection process: after the current rise rate reaches the second rise rate threshold, the nature of the fault path is first differentiated, and then a decision is made on whether the circuit breaker should be tripped accordingly; the aforementioned method of blocking the power conversion unit and controlling the circuit breaker to trip does not differentiate the nature of the fault path and trips the circuit breaker in all cases. Either method can be implemented, or they can be combined; in the combined implementation, after the current rise rate reaches the second rise rate threshold, the power conversion unit is first blocked and the nature of the fault path is differentiated. If the fault is determined to be permanent, the circuit breaker is controlled to trip; if the fault is determined to be transient, the blocking of the power conversion unit is released and it is controlled to adjust to the target output power.

[0081] Reference Figure 4 , Figure 4 This is a flowchart illustrating fault type determination and recovery according to one embodiment of the present invention. In some embodiments, such as... Figure 4 As shown, the method further includes: acquiring voltage oscillation characteristics and current oscillation characteristics on the DC side when the current rise rate is greater than or equal to a second rise rate threshold; the second rise rate threshold is greater than a first rise rate threshold; determining the fault type based on the voltage oscillation characteristics and current oscillation characteristics; controlling the circuit breaker to perform tripping when the fault type is a permanent fault; controlling the power conversion unit to adjust to the target output power when the fault type is a transient fault, and controlling the circuit breaker to perform tripping if the current rise rate is greater than or equal to the first rise rate threshold during the adjustment process.

[0082] Voltage oscillation characteristics and current oscillation characteristics refer to the frequency components, amplitude envelopes, and decay patterns of the oscillating components of DC-side voltage and current during a fault. The fault path formed by metallic contact has stable impedance. The oscillating component formed by the inductance of the DC-side circuit and the DC bus capacitance decays monotonically with circuit damping, and its frequency is concentrated. Its frequency is determined by the circuit inductance and bus capacitance, ranging from hundreds to thousands of hertz, for example, 200 Hz to 3 kHz. The impedance of the arc fault path is unstable, and its current and voltage contain broadband noise components with frequencies ranging from 1 kHz to 100 kHz, for example, 1 kHz, 10 kHz, 50 kHz, or 100 kHz. Both types of components are extracted through bandpass filtering and spectral transformation of their respective frequency bands. The resulting amplitude envelopes and frequency components are used for discrimination. The two frequency bands can partially overlap; discrimination is based on the decay pattern and spectral concentration of the oscillating components.

[0083] Accordingly, when the oscillation component exhibits monotonically decaying and its frequency components are concentrated, the fault path is determined to be a metallic contact, which is a permanent fault; when the broadband noise component is significant and its amplitude envelope deviates from the monotonically decaying law, the fault path is determined to be an arcing path. An arcing path may cease to exist after its excitation disappears, or it may persist due to carbonization of the insulating medium. Therefore, in this embodiment, after determining that it is an arcing path, the recovery process of charging and discharging power is further used as a means of verifying whether the path is still maintained.

[0084] In this embodiment, the extraction of oscillation characteristics is not used to determine whether an arc exists on the DC side. Instead, it determines whether the fault path persists after the excitation that caused it to form has disappeared, given that the current rise rate has reached the second rise rate threshold and a short circuit has been established. In other words, the object of the determination is the maintenance condition of the fault path, not the presence or absence of an arc. Therefore, if an arcing path is determined, the DC side circuit is not directly disconnected. Instead, the recovery process of the charging and discharging power is used to verify whether the path is still maintained. The circuit breaker is only controlled to trip when the current rise rate reaches the first rise rate threshold again during the verification process. Furthermore, the extraction of oscillation characteristics only begins after the current rise rate reaches the second rise rate threshold, and is not continuous during normal operation.

[0085] In a specific example, the target output power refers to the expected charging and discharging power level that the power conversion unit should return to after fault handling. This corresponds to the restored charging and discharging power under charging and discharging conditions, respectively. Its value can range from 20% to 100% of the charging and discharging power before the fault, for example, 20%, 50%, 80%, or 100%. The power conversion unit is adjusted to the target output power using a power ramp, with the ramp rate ranging from 1% / ms to 20% / ms of the rated power, for example, 1% / ms, 5% / ms, 10% / ms, or 20% / ms.

[0086] At this point, the charging and discharging power has been reduced in the aforementioned steps, and the current in the fault path is at a reduced level. The power ramp is restored gradually rather than all at once, making the restoration process itself a verification of whether the fault path still exists: if the fault path no longer exists, the current rise rate will no longer reach the first rise rate threshold during the restoration process; if the fault path still exists, the DC-side current will rapidly rise again during the power ramp, and the current rise rate will again reach the first rise rate threshold, thereby controlling the circuit breaker to trip. If the current rise rate does not reach the first rise rate threshold again during the adjustment process, the power conversion unit restores to the target output power. When the slope of the power ramp is at the lower limit of the above range, the energy injected into the fault path during this verification process is correspondingly reduced.

[0087] Therefore, this embodiment distinguishes the nature of the fault path by the attenuation law of the oscillation component and the broadband noise component, and uses the power ramp recovery process itself as a verification method to determine whether the fault path is still maintained. Compared with the method of reversing the circuit breaker at fixed time intervals after the circuit breaker is opened, the state of the fault path can be determined in the early stage of power recovery, which is beneficial to reduce the fault current flowing through the battery cluster and power device during the verification process.

[0088] In some embodiments, the method for determining the circuit breaker operating delay is further explained below.

[0089] The circuit breaker does not trip immediately after the current reaches the protection threshold, but rather after an operating delay. The length of this delay determines the trade-off between selectivity and speed of the protection. The operating delay can be determined in various ways, such as by classifying the current range, by continuous calculation using a thermal accumulation model, or by setting it according to the coordination relationship between upper and lower level protection. This embodiment uses the method of determining the delay by classifying the current range as an example.

[0090] In some embodiments, the circuit breaker further has a short-circuit protection threshold and a transient protection threshold, wherein the short-circuit protection threshold is greater than the overcurrent protection threshold and the transient protection threshold is greater than the short-circuit protection threshold; the method further includes: determining an operation delay as a first delay when the current on the DC side of the power conversion unit is greater than or equal to the overcurrent protection threshold and less than the short-circuit protection threshold; determining an operation delay as a second delay when the current on the DC side of the power conversion unit is greater than or equal to the short-circuit protection threshold and less than the transient protection threshold; the first delay is greater than the second delay; and determining the operation delay to zero when the current on the DC side is greater than or equal to the transient protection threshold.

[0091] In a specific example, the short-circuit protection threshold can be 2 to 5 times the overcurrent protection threshold, for example, 2, 3, 4 or 5 times; the instantaneous protection threshold can be 6 to 12 times the overcurrent protection threshold, for example, 6, 8, 10 or 12 times.

[0092] When the current reaches the instantaneous protection threshold, its amplitude indicates that the impedance of the fault path has dropped to an extremely low level. At this time, the action delay is set to zero, and the circuit breaker performs the tripping without any additional waiting.

[0093] Therefore, in this embodiment, the circuit breaker's operating delay corresponds to the conductor's thermal withstand capability, the through energy withstand capability, and the device's instantaneous withstand capability in the three current zones, respectively. Compared to the case where a single delay is used in all zones, the delay can be retained in the zone where the current is close to the rated value to avoid short-term overload and inrush current, while the delay can be canceled in the zone where the current is at an extremely high level. This is beneficial for the same set of protection thresholds to take into account both selectivity and speed.

[0094] In some embodiments, the method for handling the delay in the AC side voltage drop condition is further explained below.

[0095] During AC voltage dips, energy storage converters must perform low-voltage ride-through according to grid connection specifications, maintaining grid connection and outputting reactive current to the AC side to support voltage. This low-voltage ride-through requires a certain period, and if the circuit breaker trips with a conventional delay during this period, the low-voltage ride-through cannot be completed. This conflict can be resolved in various ways, such as extending the circuit breaker's operating delay, temporarily blocking the circuit breaker's tripping circuit, or having the upstream protection system assume the responsibility for disconnection during this period. This embodiment uses extending the circuit breaker's operating delay as an example.

[0096] In some embodiments, the power conversion unit further includes an AC side, and the method further includes: when the voltage on the AC side of the power conversion unit is less than a set voltage and the operating delay of the circuit breaker is greater than zero, extending the operating delay and controlling the power conversion unit to output reactive current to the AC side.

[0097] In a specific example, the set voltage can be 0.5 to 0.9 times the rated AC voltage, for example, 0.5, 0.7, 0.8, or 0.9 times. The total operating delay of the circuit breaker consists of the sum of the inherent delay and the dynamic delay, as shown in the formula T_brk=T_fix+T_dyn, where T_brk is the total operating delay in milliseconds (ms); T_fix is ​​the inherent delay in milliseconds, including the relay action time, the mechanism opening time, and the arc-extinguishing chamber arc-extinguishing delay, which is a factory-set or measured value, typically 2ms to 20ms, for example, 2ms, 5ms, 10ms, or 20ms; T_dyn is the dynamic delay in milliseconds, i.e., the aforementioned adjustable operating delay, which is an adjustable component determined in real time according to the operating state; when the operating delay is set to zero, the total operating delay equals the inherent delay. Extending the operating delay is achieved by increasing the dynamic delay.

[0098] In a specific example, the lower limit of the dynamic delay is determined based on the operating delay of the upstream protection and the operating delay of the adjacent branch protection, ensuring that the operating time of the current protection is staggered from that of the upstream and adjacent branch protections, thus maintaining protection selectivity. The upper limit of the dynamic delay is determined based on the thermal withstand time of the energy storage converter and the remaining withstand time of low-voltage ride-through, ensuring that the circuit breaker operates before the thermal withstand capability of the power devices and DC cables is exhausted, while simultaneously allowing the low-voltage ride-through to be completed. The dynamic delay takes a value between this lower and upper limit, and the extended dynamic delay can be from 100ms to 2000ms, for example, 100ms, 300ms, 1000ms, or 2000ms. This invention does not limit the specific input set used to solve the dynamic delay.

[0099] It should be noted that when the AC voltage drop is caused by a DC side fault, the DC side current rise rate simultaneously reaches either the first rise rate threshold or the second rise rate threshold. In this case, the charging / discharging power is reduced, the power conversion unit is blocked, or the circuit breaker is controlled to open according to the current rise rate threshold. When the circuit breaker opens according to the current rise rate threshold, its action delay is determined to be zero, and the condition for extending the action delay is no longer met.

[0100] Therefore, this embodiment extends the circuit breaker's operating delay during AC voltage dips to a level sufficient to accommodate the low-voltage ride-through process. This extension is constrained by both thermal withstand time and protection coordination. Compared to the case of direct tripping with conventional delay during AC faults, this is beneficial for the energy storage converter to provide reactive power support for the AC voltage while maintaining protection selectivity.

[0101] The steps described above are for clarity only. In practice, they can be combined into one step or some steps can be split into multiple steps. As long as they include the same logical relationship, they are all within the protection scope of this invention. Adding insignificant modifications or introducing insignificant designs to the algorithm or process, without changing the core design of the algorithm and process, are also within the protection scope of this invention.

[0102] Furthermore, the examples mentioned in the above embodiments can be freely combined, and any combination can be understood as an embodiment. The terms "embodiment" or "example" appearing in various locations in the specification do not necessarily refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. Those skilled in the art will understand that the embodiments described herein can be combined with other embodiments.

[0103] In summary, specific embodiments of the subject matter have been described. Other embodiments are within the scope of the appended claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require a specific or sequential order to achieve the desired result.

[0104] This invention also provides an energy storage converter for executing the aforementioned protection method for energy storage converters. The energy storage converter provided in this invention corresponds to the aforementioned protection method for energy storage converters. The actions performed by each component in the energy storage converter correspond one-to-one with the steps of the method. The parameter selection, optional extensions, and technical effects of each step described in the aforementioned method are also applicable to this energy storage converter, and will not be repeated here.

[0105] like Figure 1 As shown, the energy storage converter includes a power conversion unit, a circuit breaker electrically connected to the DC side of the power conversion unit, and may also include a converter control unit and a protection setting unit. The converter control unit is configured to acquire the sampled current of the DC side of the power conversion unit, obtain the current rise rate of the DC side from the sampled current, and reduce the charging and discharging power of the energy storage converter if the current rise rate is greater than or equal to a first rise rate threshold. The protection setting unit is configured to reduce the overcurrent protection threshold of the circuit breaker based on the reduced charging and discharging power. The converter control unit and the protection setting unit can be implemented by a processor executing program code stored in memory, or by hardware circuits such as field-programmable gate arrays or application-specific integrated circuits. They can be integrated on the same control board or located on different control boards and connected via internal signal channels.

[0106] This invention also provides an energy storage system, including the aforementioned energy storage converter. In one example, the energy storage system further includes battery clusters electrically connected to a circuit breaker in the energy storage converter, an energy management system, and a battery management system, wherein the battery management system is communicatively connected to the battery cluster management unit of each battery cluster. Corresponding to the protection method for the aforementioned energy storage converter, the parameter selection, optional extensions, and technical effect descriptions of each step in the aforementioned method are also applicable to this energy storage system and will not be repeated here.

[0107] This invention also provides an electrical device including the aforementioned energy storage converter. In one example, the electrical device includes the aforementioned energy storage converter and a load; in another example, the electrical device includes an energy storage system and a load, the energy storage system including the aforementioned energy storage converter, and the load being electrically connected to the energy storage system. The protection method for the electrical device corresponds to that of the aforementioned energy storage converter, and the parameter selection, optional extensions, and technical effects of the aforementioned method are also applicable to this electrical device, and will not be repeated here.

[0108] This invention also provides a control device that is communicatively connected to an energy storage converter. The control device is used to execute the aforementioned protection method for the energy storage converter. Corresponding to the aforementioned protection method for the energy storage converter, the parameter selection, optional extensions, and technical effects of each step in the method are also applicable to this control device and will not be repeated here. In one example, the control device is deployed as an independent controller within the energy storage cabinet or integrated inside the energy storage converter. It is communicatively connected to the power conversion unit and protection setting unit via the internal signal channel of the energy storage converter. The reduction of charging and discharging power and overcurrent protection threshold, the blocking of the power conversion unit, and the tripping of the circuit breaker are all executed through this internal signal channel. In another example, the control device is the energy management system of the energy storage system or the monitoring host of the energy storage power station. It sends setting parameters such as the reference threshold, load sensitivity coefficient, and various rise rate thresholds via a communication link. The aforementioned reduction during the fault development process is still executed internally by the energy storage converter. This invention does not limit the deployment location of the control device.

[0109] The energy storage converter, energy storage system, and electrical equipment provided in this invention can be widely applied in energy storage fields requiring high capacity and long cycle life, including large-scale energy storage power stations, industrial and commercial energy storage, and residential energy storage. The duration and amplitude of DC-side fault current are reduced, correspondingly decreasing the fault current surges experienced by the battery cluster during its lifespan, which is beneficial for maintaining the battery cluster's performance during long-term cycling. The aforementioned energy storage system can achieve large-capacity energy storage and meet the needs of long-term energy storage, enabling long-term energy storage of 4 hours or more, for example, it can be applied to energy storage scenarios such as 5 hours, 6 hours, and 8 hours. Long-term energy storage refers to the ability to continuously discharge at rated power for 4 hours or even longer, or to achieve large-scale, low-cost energy storage for several days or months.

[0110] Energy storage devices include, but are not limited to, residential energy storage cabinets, commercial energy storage cabinets, energy storage containers, energy storage racks, energy storage power stations, energy storage battery packs, or portable energy storage systems. Energy storage devices may also include energy management systems, battery management systems, and energy storage converters. Electrical devices include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, and spacecraft. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; spacecraft can include airplanes, rockets, space shuttles, and spacecraft.

[0111] Those skilled in the art will understand that the above embodiments are specific embodiments for implementing the present invention. In practical applications, several changes in form and detail may be made without departing from the spirit and scope of the present invention.

Claims

1. A protection method for an energy storage converter, characterized in that, The energy storage converter includes a power conversion unit and a circuit breaker electrically connected to the DC side of the power conversion unit. The method includes: Obtain the sampling current on the DC side of the power conversion unit; The rate of rise of the DC side current is obtained from the sampled current; If the current rise rate is greater than or equal to the first rise rate threshold, the charging and discharging power of the energy storage converter is reduced. Based on the reduced charging and discharging power, the overcurrent protection threshold of the circuit breaker is lowered.

2. The protection method according to claim 1, characterized in that, The method further includes: If the current rise rate is greater than or equal to the second rise rate threshold, the power conversion unit is locked; the second rise rate threshold is greater than the first rise rate threshold. Control the circuit breaker to perform the tripping operation.

3. The protection method according to claim 1, characterized in that, The method further includes: If the current rise rate is greater than or equal to the second rise rate threshold, the voltage oscillation characteristics and current oscillation characteristics on the DC side are obtained; the second rise rate threshold is greater than the first rise rate threshold. The fault type is determined based on the voltage oscillation characteristics and the current oscillation characteristics; In the event that the fault type is a permanent fault, the circuit breaker is controlled to trip. In the case of a transient fault, the power conversion unit is controlled to adjust to the target output power. If the current rise rate is greater than or equal to the first rise rate threshold during the adjustment process, the circuit breaker is controlled to open.

4. The protection method according to claim 1, characterized in that, The circuit breaker is electrically connected to the battery cluster; the method further includes: Obtain the state of charge and cell temperature of the battery cluster; The maximum permissible instantaneous current of the battery cluster is determined based on the state of charge and the cell temperature. The battery protection threshold is determined based on the maximum allowable instantaneous current and the charging / discharging power. Wherein, in the step of lowering the overcurrent protection threshold of the circuit breaker according to the lowered charging and discharging power, the lowered overcurrent protection threshold of the circuit breaker is not greater than the battery protection threshold.

5. The protection method according to claim 1, characterized in that, The circuit breaker further has a short-circuit protection threshold and a transient protection threshold, wherein the short-circuit protection threshold is greater than the overcurrent protection threshold, and the transient protection threshold is greater than the short-circuit protection threshold; the method further includes: If the current on the DC side of the power conversion unit is greater than or equal to the overcurrent protection threshold and less than the short-circuit protection threshold, the action delay is determined as the first delay. If the current on the DC side of the power conversion unit is greater than or equal to the short-circuit protection threshold and less than the instantaneous protection threshold, the action delay is determined as the second delay; the first delay is greater than the second delay. If the current on the DC side is greater than or equal to the instantaneous protection threshold, the action delay is set to zero.

6. The protection method according to claim 5, characterized in that, The power conversion unit also has an AC side, and the method further includes: If the voltage on the AC side of the power conversion unit is less than the set voltage and the operating delay of the circuit breaker is greater than zero, the operating delay is extended and the power conversion unit is controlled to output reactive current to the AC side.

7. The protection method according to claim 1, characterized in that, The method further includes: Based on the sampled current, determine the effective value of the current on the DC side; When the current rise rate is greater than or equal to a first rise rate threshold, reducing the charging and discharging power of the energy storage converter includes: If the current rise rate is greater than or equal to the first rise rate threshold and the effective current value is greater than or equal to the first current threshold, the charging and discharging power of the energy storage converter is reduced.

8. The protection method according to claim 1, characterized in that, The reduction of the charging and discharging power of the energy storage converter includes: If the current rise rate is greater than or equal to the third rise rate threshold, the charging and discharging power is reduced by the first reduction rate. If the current rise rate is greater than or equal to the first rise rate threshold and less than the third rise rate threshold, the charging and discharging power is reduced by the second reduction rate. The third rise rate threshold is greater than the first rise rate threshold, and the first drop is greater than the second drop.

9. The protection method according to claim 1, characterized in that, The method further includes: When the current rise rate is greater than or equal to the first rise rate threshold, the voltage on the DC side of the power conversion unit and the harmonic distortion rate on the AC side of the power conversion unit are obtained. The current rise rate, the DC side voltage, and the harmonic distortion rate are weighted according to a set weight to obtain the action confidence score; the action confidence score characterizes the degree of confidence that a short-circuit fault will occur. If the confidence level of the action is greater than or equal to the confidence threshold, the charging and discharging power is reduced.

10. The protection method according to claim 1, characterized in that, The sampling frequency of the sampled current is greater than the current sampling frequency of the circuit breaker.

11. An energy storage converter, characterized in that, The energy storage converter is used to perform the protection method for the energy storage converter as described in any one of claims 1 to 10.

12. An energy storage system, characterized in that, Including the energy storage converter as described in claim 11.

13. An electrical appliance, characterized in that, Including the energy storage converter as described in claim 11.

14. A control device, characterized in that, The control device is communicatively connected to the energy storage converter, and the control device is used to execute the protection method of the energy storage converter as described in any one of claims 1 to 10.

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