Zero-power parallel operation method for energy storage converters

Abstract

The invention discloses a zero-power parallel operation method of an energy storage converter. An energy storage converter is generally controlled by alternating-current side power, when the energy storage converter in parallel operation operates at zero power, pulses cannot be directly closed, otherwise, alternating-current capacitance current unbalance occurs, the direct-current zero-current control can be switched, and the pulses are closed until other stations operate at zero power. Zero-power operation states of other energy storage converters can be judged in a multi-machine communication mode generally, and the operation states of other energy storage converters are obtained through a method of opening and closing pulses at regular time and actively detecting the magnitude of zero-sequence current. On the basis that hardware cost is not increased, the problems of battery overcharge and overdischarge caused by alternating current side filter capacitor current imbalance and alternating current side power control deviation during zero-power parallel operation of the energy storage converter are effectively solved through a software algorithm, the service life of the capacitor is prolonged, and system operation is stabilized.

Description

Technical Field

[0001] The invention relates to a zero-power parallel operation method for an energy storage converter, and belongs to the technical field of energy storage converter control. Background Art

[0002] When a large-capacity centralized energy storage converter (PCS, Power Conversion System) is running on site, generally at least two energy storage converters are operated in parallel under one transformer, the AC sides of the energy storage converters are directly connected in parallel, and the DC sides are connected to independent battery compartments. The converter charges and discharges the battery according to the active and reactive power instructions issued by the energy management system (EMS, Energy Management System) combined with the information provided by the battery management system (BMS, Battery Management System).

[0003] Due to the difference in batteries, the batteries will not be fully charged or discharged at the same time. One or more energy storage converters are working in the charging or discharging state, and the pulse is on. The remaining energy storage converters are turned to zero power and the pulse is off because the batteries are full or empty. The impedance characteristics presented to the outside by the energy storage converter in these two states are inconsistent. At this time, there will be circulating currents between the energy storage converters running in parallel, resulting in unbalanced and overloaded currents on the AC side filter capacitors, affecting the life of the capacitors. At the same time, the AC side power control of the energy storage converter working in the charging or discharging state will deviate. In severe cases, the battery charging will be changed to discharging, and discharging will be changed to charging, and the normal charging or discharging function cannot be completed. In order to ensure the balance of the filter capacitor current, it is usually necessary to sample the three-phase capacitor current and use the method of suppressing zero-sequence current to reduce the low-frequency circulating current. This will increase the hardware cost and the suppression effect of high-frequency circulating current is not good. Summary of the invention

[0004] The technical problem to be solved by the present invention is: without increasing the hardware cost, a software control method is used to ensure that the three-phase balance of the AC side filter capacitor current is achieved when one or more of the energy storage converters operating in parallel is running at zero power due to full or empty batteries.

[0005] In order to solve the above technical problems, the technical solution of the present invention provides a method for zero-power parallel operation of energy storage converters, wherein at least two or more energy storage converters are operated in parallel, and each energy storage converter operating in parallel adopts AC power control according to the active and reactive power instructions issued by the energy management system to maintain AC side power stability, characterized in that the method comprises the following steps:

[0006] When the active and reactive power command values ​​are zero, the current energy storage converter switches from AC power control to DC zero current control, and opens and closes pulses at regular intervals to determine the operating status of other energy storage converters:

[0007] When the pulse is turned off: if the zero-sequence current is less than the set threshold and can be maintained for a period of time, it is considered that the energy storage converters running in parallel are all operating at zero power, and the pulse is turned off. At this time, the current of the three-phase filter capacitor remains balanced; otherwise, at least one energy storage converter is in the charging or discharging state, at this time, the pulse is turned on.

[0008] Preferably, the zero-sequence current is calculated using the following formula:

[0009] I0=(I a +I b +I c ) / 3

[0010] Where I0 is the zero-sequence current, I a ,I b ,I c is the instantaneous value of three-phase alternating current.

[0011] Preferably, the method specifically comprises the following steps:

[0012] Step S1: Determine whether the active and reactive power command values ​​are zero:

[0013] If the active and reactive power command values ​​are zero, the machine operation state is set to hot standby state, switched to DC zero current control, and step S2 is executed;

[0014] If the active and reactive power command values ​​are not zero, the machine operation state is set to charging or discharging state, and the AC power control method is executed;

[0015] Step S2, adopting DC zero current control, and executing step S3, wherein the DC zero current control method is to take the DC current as the control target and control the DC current to zero, and the battery is neither charged nor discharged at this time;

[0016] Step S3, turn on the pulse at a fixed time, and execute step S4;

[0017] Step S4, turn off the pulse at a fixed time, and execute step S5;

[0018] Step S5, calculate the zero-sequence current. If the zero-sequence current is less than the threshold, execute step S6, otherwise return to step S1;

[0019] Step S6: turn off the pulse and keep the AC side circuit breaker closed.

[0020] Preferably, in step S3, the opening pulse time is controlled at the second level to ensure that there is enough time to stabilize the DC current.

[0021] Preferably, in step S4, the closing pulse time is controlled at the second level to ensure sufficient time for zero-sequence current calculation and judgment.

[0022] Energy storage converters usually use AC side power control. When parallel-operated energy storage converters are in zero-power operation, the pulse cannot be directly turned off, otherwise there will be an imbalance in the AC capacitor current. The present invention proposes to switch to DC zero current control until the other converters are in zero-power operation, and then the pulse is turned off. The zero-power operation state of other energy storage converters can usually be obtained by multi-machine communication. The present invention proposes to use a method of timing the opening and closing of pulses and actively detecting the magnitude of zero-sequence current to obtain the operating state of other energy storage converters.

[0023] Without increasing the hardware cost, the present invention effectively solves the problems of unbalanced current of the AC side filter capacitor and over-charge and over-discharge of the battery caused by AC side power control deviation when the energy storage converter is operated in zero-power parallel through software algorithm, thereby extending the service life of the capacitor and stabilizing the operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of parallel operation of energy storage converters provided in an embodiment of the present invention;

[0025] Figure 2 A flow chart of a control method for an energy storage converter provided by an embodiment of the present invention;

[0026] Figure 3 An AC power control block diagram provided by an embodiment of the present invention;

[0027] Figure 4 A block diagram of a DC zero current control provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0029] like Figure 1 As shown, the parallel operation of energy storage converters includes: at least two or more energy storage converters are operated in parallel, and the DC side of each energy storage converter is connected to the battery compartment, and the AC side is connected to the transformer through LC filtering and AC circuit breaker. Each energy storage converter operating in parallel adopts AC power control according to the active and reactive power instructions issued by the energy management system to maintain the stability of the AC side power. At this time, the pulse is turned on and the current of the AC filter capacitor maintains a three-phase balance.

[0030] based on Figure 1 , a zero-power parallel operation method of an energy storage converter disclosed in an embodiment of the present invention comprises the following steps:

[0031] When the active and reactive power command value is zero, the energy storage converter switches from AC power control to DC zero current control, and opens and closes pulses at a fixed time. The DC zero current control refers to a control method that controls the current on the DC side of the energy storage converter to be zero. The control method switches from AC power control to DC zero current control to avoid the situation where the DC side current is not zero due to sampling errors of the energy storage converter itself, and the battery continues to charge or discharge when it is full or empty.

[0032] Calculate the zero-sequence current based on the sampled three-phase AC current.

[0033] Calculation method of zero sequence current I0: I0=(I a +I b +I c ) / 3, where I a ,I b ,I c is the instantaneous value of three-phase alternating current.

[0034] When the pulse is turned off: the zero-sequence current is less than the set threshold and can be maintained continuously for a period of time, that is, it is considered that the energy storage converters running in parallel are all operating at zero power, and the pulse can be turned off. At this time, the current of the three-phase filter capacitor remains balanced; otherwise, at least one energy storage converter is in the charging or discharging state, at this time, the pulse needs to be turned on.

[0035] When the energy storage converters running in parallel are all in zero power operation, they enter the hot standby state, that is, the pulse is turned off, and the AC circuit breaker remains closed and does not disconnect from the grid. The zero power operation means that the active and reactive power instructions issued by the energy management system are zero, requiring the active power and reactive power on the AC side to be zero, that is, no power is output to the outside.

[0036] When the energy storage converters operating in parallel are all charging or discharging, the original control method and operating state are maintained, that is, AC power control, pulse opening. The AC power control refers to a control method for controlling the power on the AC side of the energy storage converter to be a constant value.

[0037] The energy storage converters running in parallel will open the pulses during zero-power operation. At this time, the external equivalent impedance of the energy storage converters remains consistent, thereby preventing the circulating current caused by inconsistent impedance from flowing inside the energy storage converters running in parallel.

[0038] After the energy storage converter switches to DC zero current control, the operating status of other energy storage converters is determined by timing the opening and closing of pulses. When the zero-sequence current when the pulse is closed is less than the threshold, it can be determined that all energy storage converters are in zero power operation, and the pulse is turned off to reduce losses. In practical applications, turning off the pulse and keeping the AC side circuit breaker closed is a common low-power operation mode for energy storage power stations.

[0039] Based on the above principles, Figure 2 As shown, the present invention provides a specific implementation method comprising the following steps:

[0040] Step S1, judging whether the active and reactive power command values ​​are zero.

[0041] If the active and reactive power command values ​​are zero, the machine operation state is set to hot standby state, switched to DC zero current control, and step S2 is executed.

[0042] If the active and reactive power command values ​​are not zero, the operating state of the machine is set to charging or discharging state, and the AC power control method is executed.

[0043] The AC power control method refers to the energy storage converter controlling the active power and reactive power on the AC side according to the active and reactive power command values ​​issued by the energy management system during normal charging or discharging, ensuring that the active power and reactive power on the AC side are consistent with the command values. The control method is as follows: Figure 3 shown.

[0044] Step S2: adopt DC zero current control and execute step S3.

[0045] The DC zero current control method is to take the DC current as the control target and control the DC current to zero. At this time, the battery is neither charged nor discharged. The control method is as follows: Figure 4 shown.

[0046] Step S3, turn on the pulse at a fixed time, and execute step S4.

[0047] The opening pulse time is controlled in seconds to ensure that there is enough time for the DC current to stabilize.

[0048] Step S4, turn off the pulse at a fixed time, and execute step S5.

[0049] The closing pulse time is controlled in seconds to ensure sufficient time for zero-sequence current calculation and judgment.

[0050] Step S5, calculate the zero-sequence current. If the zero-sequence current is less than the threshold, execute step S6, otherwise return to step S1.

[0051] Step S6: turn off the pulse and keep the AC side circuit breaker closed.

Claims

1. A method for zero-power parallel operation of energy storage converters, wherein at least two energy storage converters are operated in parallel, and each energy storage converter operating in parallel adopts AC power control according to active and reactive power instructions issued by an energy management system to maintain AC side power stability, characterized in that: The method comprises the following steps: When the active and reactive power command values ​​are zero, the current energy storage converter switches from AC power control to DC zero current control, and opens and closes pulses at regular intervals to determine the operating status of other energy storage converters: When the pulse is turned off: if the zero-sequence current is less than the set threshold and can be maintained for a period of time, it is considered that the energy storage converters running in parallel are all operating at zero power, and the pulse is turned off. At this time, the current of the three-phase filter capacitor remains balanced; otherwise, at least one energy storage converter is in the charging or discharging state, at this time, the pulse is turned on.

2. A method for zero-power parallel operation of energy storage converters as claimed in claim 1, characterized in that: The zero-sequence current is calculated using the following formula: I0=(I a +I b +I c ) / 3 Where I0 is the zero-sequence current, I a ,I b ,I c is the instantaneous value of three-phase alternating current.

3. A method for zero-power parallel operation of energy storage converters as claimed in claim 1, characterized in that: The method specifically comprises the following steps: Step S1: Determine whether the active and reactive power command values ​​are zero: If the active and reactive power command values ​​are zero, the machine operation state is set to hot standby state, switched to DC zero current control, and step S2 is executed; If the active and reactive power command values ​​are not zero, the machine operation state is set to charging or discharging state, and the AC power control method is executed; Step S2, adopting DC zero current control, and executing step S3, wherein the DC zero current control method is to take the DC current as the control target and control the DC current to zero, and the battery is neither charged nor discharged at this time; Step S3, turn on the pulse at a fixed time, and execute step S4; Step S4, turn off the pulse at a fixed time, and execute step S5; Step S5, calculate the zero-sequence current. If the zero-sequence current is less than the threshold, execute step S6, otherwise return to step S1; Step S6: turn off the pulse and keep the AC side circuit breaker closed.

4. A method for zero-power parallel operation of energy storage converters as claimed in claim 3, characterized in that: In step S3, the opening pulse time is controlled in seconds to ensure that there is enough time to stabilize the DC current.

5. A method for zero-power parallel operation of energy storage converters as claimed in claim 3, characterized in that: In step S4, the closing pulse time is controlled at the second level to ensure sufficient time for zero-sequence current calculation and judgment.

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