Variable speed hydroelectric generating set low voltage ride through subsection control method, system, equipment and medium

Abstract

The invention discloses a variable-speed hydroelectric generating set low-voltage ride-through segmented control method, system, equipment and medium, and belongs to the technical field of new energy power generation grid connection, and the method comprises the steps: according to the detected voltage of a grid connection point, taking whether the output active power of a grid-side converter during low-voltage ride-through is kept unchanged and whether a crowbar circuit is put into a segment basis; dividing a fault voltage interval into three sections; determining a reactive current proportionality coefficient calculation expression of each voltage interval according to a low-voltage ride-through requirement, a working condition of a hydroelectric generating set before a fault and a current limiting value constraint condition of a converter; and determining a grid-side converter, unloading circuit and crowbar cooperative control method according to each voltage interval. According to the method, the supporting force of the variable-speed hydroelectric generating set on the power grid voltage when the fault is shallow can be increased, meanwhile, the output active power is considered to the maximum extent, the crowbar is put into a reasonable voltage interval, and stable operation of the power grid is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy power generation and grid connection, and in particular to a low voltage ride-through segmented control method, system, equipment and medium for a variable speed hydropower unit. Background Art

[0002] As the proportion of renewable energy sources, such as wind power and photovoltaics, continues to increase, hydropower, as a low-cost clean energy source, plays a crucial role in current power system development due to its critical roles in peak-load shifting, frequency regulation, and emergency backup. When variable-speed hydropower units face low voltage ride-through (LVRT) issues, they can potentially disconnect from the grid if appropriate control measures are not implemented. Therefore, variable-speed hydropower units must also possess a certain degree of LVRT capability to ensure stable grid-connected operation.

[0003] Existing technologies utilize a fixed reactive current proportional coefficient to increase reactive power generated by the grid-side converter during low-voltage ride-through (LVRT) of wind turbines, while also deploying a crowbar circuit on the rotor side to prevent overcurrent in the generator-side converter. However, this strategy is ineffective in addressing varying degrees of grid faults. In particular, when the fault is relatively shallow, the fixed coefficient strategy cannot fully utilize the reactive power support capacity of the grid-side converter, hindering grid voltage recovery. Therefore, it is necessary to segment the fault voltage and propose a control strategy that varies the reactive current proportional coefficient for each interval. Crowbars can also be deployed in appropriate intervals to protect the generator-side converter while addressing the issue of the negative impact of crowbar deployment on generator-side voltage recovery when the fault is relatively shallow. Summary of the Invention

[0004] In view of the above-mentioned problems, the present invention is proposed.

[0005] Therefore, the technical problem solved by the present invention is: how to solve the problem that the existing technology uses the grid-side converter of the wind turbine to increase the reactive power by a fixed reactive current proportional coefficient during low voltage ride-through, and at the same time equips the rotor side with a crowbar circuit to prevent overcurrent in the machine-side converter, but cannot cope well with grid faults of different degrees, especially when the fault degree is relatively shallow, the fixed coefficient strategy cannot fully exert the reactive support capability of the grid-side converter, which is not conducive to the recovery of the grid voltage.

[0006] To solve the above technical problems, the present invention provides the following technical solutions: a segmented control method for low voltage ride-through of a variable-speed hydropower unit, comprising the following steps: dividing the fault voltage interval into three segments based on the detected voltage of the grid-connected point and whether the active power output of the grid-side converter remains unchanged during the low voltage ride-through period and whether the crowbar circuit is put into operation; determining a calculation expression for the reactive current proportional coefficient of each voltage interval based on the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault, and the current limit constraint of the converter; and determining a coordinated control method for the grid-side converter, the unloading circuit, and the crowbar according to each voltage interval.

[0007] As a preferred solution of the low voltage ride-through segmented control method of a variable speed hydropower unit described in the present invention, wherein: the fault voltage interval is divided into three segments including low voltage ride-through segment 1, low voltage ride-through segment 2 and low voltage ride-through segment 3; low voltage ride-through segment 1 includes k2∈(k 21max , k max ], where k2 is the per-unit voltage of the hydropower unit grid connection point, k 21max is the voltage of the crowbar input, k max is the upper limit of the standard voltage for low voltage ride-through of hydropower units; the rotor-side converter is protected by a crowbar to avoid overcurrent under different outputs. Assuming that the generator is fully powered before the fault, the crowbar input voltage k 21max Expressed as:

[0008]

[0009] T s ′=L s ′R s

[0010] T r ′=L r ′R r

[0011] Among them, I rmax is the maximum current on the rotor side, I RSC is the current limit value of the rotor-side converter, L m is the mutual inductance between the stator and rotor in the dq coordinate system, L s is the stator self-inductance in the dq coordinate system, ω s is the synchronous speed, L r ′ is the rotor equivalent inductance, L r is the rotor self-inductance in the dq coordinate system, ψ r is the rotor flux amplitude, T is the system period, T s ′ is the time decay constant of the DC component of the stator flux, T r ′ is the time decay constant of the DC component of the rotor flux, L s ′ is the stator equivalent inductance, R s is the stator resistance, R r is the rotor resistance; the crowbar input voltage k 21max The meaning is that when the grid voltage is higher than k 21max When the doubly-fed induction motor is fully powered, the maximum value of the rotor current after the fault does not exceed the current limit of the rotor-side converter, and the crowbar does not need to be put into operation.

[0012] As a preferred solution of the low voltage ride-through segmented control method of a variable speed hydroelectric generator set described in the present invention, wherein: the low voltage ride-through segment 2 includes k2∈(k 22min , k 21max ], where k 22min To consider the grid-side converter current constraint, low voltage ride-through requires k1 ≥ k 1min Compared with the working condition before the fault I gd =I gd0+ The critical voltage during the post-low voltage ride-through period is expressed as:

[0013]

[0014] Among them, I gmax is the per-unit value of the AC side output current limit of the grid-side converter, I gd0+ is the per-unit value of the AC side active current output by the grid-side converter before the fault, I gd is the active current output by the AC side of the grid-side converter, k 1min is the lower limit of the reactive current proportional coefficient of the hydropower low voltage ride-through standard; the critical voltage k 22min The meaning is that under the premise of meeting the grid-side converter current constraint and low voltage ride-through requirements, the grid-side converter output active current remains unchanged and the minimum voltage that the hydropower unit rides through is obtained by substituting various parameters into the calculation. 21max >k 22min .

[0015] As a preferred solution of the segmented control method for low voltage ride-through of a variable speed hydroelectric generator set described in the present invention, wherein: the low voltage ride-through segment three includes k2∈[max{k 2min , k min}, k 22min ], where k min The lower limit of the standard voltage for low voltage ride through of hydropower units, k 2min To consider the low voltage ride through requirement k1 ≥ k 1min and the grid-side converter current constraint ΔI gq ≤I gmax , the limit voltage during low voltage ride-through is expressed as:

[0016]

[0017] Among them, ΔI gq The reactive current per unit value generated by the hydropower unit; the limit voltage k 2minThe meaning is to reduce the grid-side converter output active current and the minimum voltage for hydropower units to ride through, while meeting the grid-side converter current constraint and low voltage ride-through requirements. The voltage interval is divided according to whether the grid-side converter output active current remains unchanged during low voltage ride-through and whether the crowbar circuit is activated. The fault voltage interval during low voltage ride-through is divided into three sections, which are expressed as follows:

[0018]

[0019] Among them, Crowbar is a crowbar.

[0020] As a preferred solution of the segmented control method for low voltage ride-through of a variable speed hydropower unit described in the present invention, wherein: according to the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault and the current limit constraint of the converter, the calculation expression of the reactive current proportional coefficient of each voltage interval is determined, including k2∈(k 22min , k max ], considering the grid-side converter current limit and the operating conditions of the hydropower unit before the fault, the reactive current proportional coefficient is expressed as:

[0021]

[0022] Among them, k 11 is the value of the reactive current proportional coefficient under the grid-side converter current constraint; in order to meet the low voltage ride-through requirement, k 1min ≤k1≤k 1max Constraints, where k 1max = The upper limit of the reactive current proportional coefficient of the hydropower low voltage ride through standard, so the reactive current proportional coefficient of low voltage ride through section 1 and low voltage ride through section 2 is k1=min{k 11 , k 1max}; Low voltage ride through section three k2∈[max{k 2min , k min}, k 22min ], the hydropower unit maintains k1=k 1max Under the premise of meeting the low voltage ride-through requirements, the reactive current is increased and the output active power is taken into account to the greatest extent. After the fault is over, the grid voltage is restored. If k 2min >k min This means that simply relying on the hydropower generator grid-side converter to increase active current cannot meet the low voltage ride-through requirements. An additional reactive power compensation device is needed to output a certain amount of reactive power to ensure that the grid support capacity requirements of the unit are met.

[0023] As a preferred solution of the low voltage ride-through segmented control method of a variable speed hydroelectric generator set described in the present invention, wherein: the grid-side converter, unloading circuit and crowbar coordinated control method are determined according to each voltage interval, including: when the grid connection point voltage drops, the speed regulator of the variable speed hydroelectric generator set reduces the guide vane opening according to the speed instruction, controls the flow of water entering the turbine runner, reduces the output power of the turbine, and the guide vane opening of the speed regulator of the variable speed hydroelectric generator set and the rotor mechanical speed deviation Δω m The transfer function between is expressed as:

[0024]

[0025] Where G(s) is the transfer function, y is the guide vane opening of the variable speed hydropower unit governor, T y is the response time of the main relay, K p , K i and K d are proportional, integral, and differential coefficients, respectively, T d is the differential attenuation constant, Δω m is the rotor mechanical speed deviation; during the low voltage ride-through process of the variable-speed hydropower unit, it is necessary to rely on the coordinated control of the grid-side converter, unloading circuit and crowbar to achieve rapid response and power balance and successfully carry out low voltage ride-through.

[0026] As a preferred solution of the low voltage ride-through segmented control method of a variable speed hydropower unit described in the present invention, wherein: the method for determining the coordinated control of the grid-side converter, the unloading circuit and the crowbar according to each voltage interval also includes: low voltage ride-through segment 1, the grid-side converter outputs the active current before the fault, and at the same time uses k1=min{k 11 , k 1max} proportional coefficient increases reactive current, giving full play to the reactive support capability of the grid-side converter. The crowbar does not need to be put into operation, and the rotor-side converter current will not reach the limit. The unloading circuit is put into operation when the DC bus voltage is greater than or equal to the DC bus voltage limit, consuming surplus power and protecting the DC bus. In the second low voltage ride-through stage, the grid-side converter outputs the active current before the fault, and at the same time, with k1=min{k 11 , k 1max The proportional coefficient of} increases the reactive current and gives full play to the reactive support capability of the grid-side converter. The crowbar needs to be put into use to prevent overcurrent on the rotor side. The unloading circuit is put into use when the DC bus voltage is greater than or equal to the DC bus voltage limit to consume surplus power and protect the DC bus. In the low voltage ride-through section three, the grid-side converter reduces the output active current and at the same time uses k1=k 1min The proportional coefficient increases the reactive current to meet the grid-side converter current constraint and low voltage ride-through requirements. The crowbar needs to be activated to prevent overcurrent on the rotor side. The unloading circuit is activated when the DC bus voltage is greater than or equal to the DC bus voltage limit to consume surplus power and protect the DC bus.

[0027] Another object of the present invention is to provide a segmented control system for low voltage ride-through of a variable speed hydropower unit.

[0028] In order to solve the above technical problems, the present invention provides the following technical solutions: a segmented control system for low voltage ride-through of a variable-speed hydropower unit, comprising: a variable reactive current proportional coefficient module and a collaborative control module; the variable reactive current proportional coefficient module is configured to establish a reactive current proportional coefficient expression based on the fault voltage range, taking into account the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault and the converter current limit constraints; the collaborative control module is configured to establish a collaborative problem between each converter and additional hardware; wherein the converter output is determined by the variable reactive proportional coefficient module, whether the crowbar is put into use is determined by the calculated voltage range, and the unloading circuit working range is determined according to the DC bus voltage limit.

[0029] The present invention provides a computer device, comprising a memory and a processor, wherein the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of the segmented control method for low voltage ride-through of a variable-speed hydropower unit are implemented.

[0030] The present invention provides a computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the segmented control method for low voltage ride-through of a variable-speed hydropower unit are implemented.

[0031] The beneficial effects of the present invention are as follows: the present invention can increase the support strength of the variable-speed hydropower unit on the grid voltage when the fault is relatively shallow, while taking into account the output active power to the greatest extent possible, and input the crowbar in a reasonable voltage range to ensure the stable operation of the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 The present invention provides an overall flow chart of a segmented control method for low voltage ride-through of a variable-speed hydropower unit according to an embodiment of the present invention.

[0034] Figure 2 A structural diagram of a simulation system for a segmented control method for low voltage ride-through of a variable-speed hydropower unit provided in one embodiment of the present invention.

[0035] Figure 3The output power value of a variable-speed hydropower unit provided by an embodiment of the present invention is a segmented control method for low voltage ride-through of a variable-speed hydropower unit.

[0036] Figure 4 This is a DC bus voltage value of a segmented control method for low voltage ride-through of a variable-speed hydropower unit provided by an embodiment of the present invention.

[0037] Figure 5 The output power value of a variable-speed hydropower unit provided by an embodiment of the present invention is a segmented control method for low voltage ride-through of a variable-speed hydropower unit.

[0038] Figure 6 This is a DC bus voltage value of a segmented control method for low voltage ride-through of a variable-speed hydropower unit provided by an embodiment of the present invention.

[0039] Figure 7 The output power value of a variable-speed hydropower unit provided by an embodiment of the present invention is a segmented control method for low voltage ride-through of a variable-speed hydropower unit.

[0040] Figure 8 This is a DC bus voltage value of a segmented control method for low voltage ride-through of a variable-speed hydropower unit provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0041] To make the above-mentioned objects, features, and advantages of the present invention more clearly understood, the following detailed description of the specific embodiments of the present invention is given in conjunction with the accompanying drawings. It is obvious that the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in this field without creative work should fall within the scope of protection of the present invention.

[0042] Example 1, with reference to Figure 1 and Figure 2 , which is the first embodiment of the present invention, provides a segmented control method for low voltage ride-through of a variable-speed hydropower unit, including: dividing the fault voltage interval into three segments based on the detected voltage of the grid-connected point and whether the output active power of the grid-side converter remains unchanged during the low voltage ride-through period and whether the crowbar circuit is put into use as the segmentation basis; determining the reactive current proportional coefficient calculation expression for each voltage interval based on the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault, and the converter current limit and other constraints; and determining the coordinated control method of the grid-side converter, the unloading circuit, and the crowbar according to each voltage interval.

[0043] In order to fully tap the reactive support capability of variable-speed hydropower units while taking into account the output active power to the greatest extent, and give play to their flexible adjustment advantages, so that the hydropower units can change the value of the reactive current proportional coefficient according to the voltage drop depth of the grid connection point, the pre-fault operating conditions, the GSC current limit, etc., and increase the support for the grid voltage, this implementation provides a segmented control method for low voltage ride-through of variable-speed hydropower units.

[0044] like Figure 2 As shown, this embodiment is based on a single-machine infinite system. The main parameters of this embodiment are set as follows: the capacity of the variable-speed hydropower unit is 200MVA, the frequency is 50Hz, the rated value of the outlet voltage is 13.8kV, and the active power reference value in steady state is 0.7pu.

[0045] S1. Based on the detected voltage at the grid connection point, the fault voltage range is divided into three segments based on whether the grid-side converter output active power remains unchanged during the low voltage ride-through period and whether the crowbar circuit is activated.

[0046] According to whether the active power output of the converter on the grid side of the hydropower generator remains unchanged during the voltage ride-through period and whether the crowbar circuit is put into operation, the per-unit value k2 of the grid-connected point voltage of the hydropower generator is divided into three sections: low voltage ride-through section 1, low voltage ride-through section 2 and low voltage ride-through section 3.

[0047] Low voltage ride-through section k2∈(k 21max , k max ], where k2 is the per-unit voltage of the hydropower unit grid connection point, k 21max is the voltage of the crowbar input, k max The upper limit of the low voltage ride through voltage of the hydropower generator set is 0.9.

[0048] In order to ensure that the rotor-side converter can be protected by the crowbar to avoid overcurrent at different outputs, assuming that the generator is fully powered before the fault, the crowbar input voltage k 21max The value of can be obtained by the following formula:

[0049]

[0050] T s ′=L s ′R s

[0051] T r ′=L r ′R r

[0052] Among them, I rmax is the maximum current on the rotor side, I RSCis the current limit value of the rotor side converter, which is 1.5 in this embodiment. m is the mutual inductance between the stator and rotor in the dq coordinate system, L s is the stator self-inductance in the dq coordinate system, ω s is the synchronous speed, L r ′ is the rotor equivalent inductance, L r is the rotor self-inductance in the dq coordinate system, ψ r is the rotor flux amplitude, T is the system period, T s ′ is the time decay constant of the DC component of the stator flux, T r ′ is the time decay constant of the DC component of the rotor flux, L s ′ is the stator equivalent inductance, R s is the stator resistance, R r is the rotor resistance.

[0053] I rmax =I RSC This is a critical case. When this equation holds true, it is the worst fault case that can prevent the rotor-side converter from overcurrent without the crowbar. If the fault is more serious (k2 is smaller), the current will exceed I without the crowbar. RSC Damage to the rotor side converter. The value of k2 is the crowbar input voltage k 21max The value of .

[0054] Crowbar input voltage k 21max The meaning is that when the grid voltage is higher than k 21max When the doubly-fed induction motor is fully powered, the maximum value of the rotor current after the fault does not exceed the current limit of the rotor-side converter, and the crowbar does not need to be put into operation.

[0055] Low voltage ride through section 2 k2∈(k 22min , k 21max ], where k 22min To consider the grid-side converter current constraint, low voltage ride-through requires k1 ≥ k 1min Compared with the working condition before the fault I gd =I gd0+ The critical voltage during the post-low voltage ride-through period is expressed as:

[0056]

[0057] Among them, I gmax is the per-unit value of the AC side output current limit of the grid-side converter, I gd0+ is the per-unit value of the AC side active current output by the grid-side converter before the fault, I gd is the active current output by the AC side of the grid-side converter, k 1min It is the lower limit of the reactive current proportional coefficient of the hydropower low voltage ride through standard.

[0058] For k 1min and k 1max The lower and upper limits of the reactive current proportional coefficient of the hydropower low voltage ride-through standard are referenced to the requirements for k1 in the wind power ride-through requirements: greater than or equal to 1.5 and less than or equal to 3. In this embodiment, k 1min Take 1.5, k 1max Take 3.

[0059] k1 is the reactive current proportional coefficient, which is proportional to the reactive current ΔI generated during the fault period. gq is directly proportional to, the specific formula is:

[0060] ΔI gq =k1(k max -k2)

[0061] Critical voltage k 22min It means the lowest voltage that the hydropower unit can ride through while keeping the grid-side converter output active current unchanged, under the premise of meeting the grid-side converter current constraint and low voltage ride-through requirements.

[0062] Substitute each parameter into the calculation to get k 21max >k 22min .

[0063] Low voltage ride through section three k2∈[max{k 2min , k min}, k 22min ], where k min The lower limit of the standard voltage for low voltage ride through of hydropower units, k 2min To consider the low voltage ride through requirement k1 ≥ k 1min and the grid-side converter current constraint ΔI gq ≤I gmax , the limit voltage during low voltage ride-through. In this embodiment, the upper limit of the standard voltage for low voltage ride-through of hydropower units is set with reference to the standard voltage for low voltage ride-through of wind turbine units, and is set to 0.2. 2min Expressed as:

[0064]

[0065] Among them, ΔI gq It is the unit value of reactive current generated by the hydropower unit.

[0066] Limit voltage k 2min The meaning is to reduce the grid-side converter output active current and the minimum voltage that the hydropower unit rides through, while meeting the grid-side converter current constraint and low voltage ride-through requirements.

[0067] The voltage interval is divided based on whether the grid-side converter output active power remains unchanged during low voltage ride-through and whether the crowbar circuit is activated. The fault voltage interval during low voltage ride-through is divided into three sections, which are expressed as follows:

[0068]

[0069] Among them, Crowbar is a crowbar.

[0070] S2. Determine the calculation expression for the reactive current proportional coefficient in each voltage range based on the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault, the converter current limit and other constraints.

[0071] k2∈(k 22min , k max ], considering the grid-side converter current limit and the operating conditions of the hydropower unit before the fault, the reactive current proportional coefficient is expressed as:

[0072]

[0073] Among them, k 11 is the value of the reactive current proportional coefficient under the grid-side converter current constraint;

[0074] In order to meet the low voltage ride through requirement of k 1min ≤k1≤k 1max Constraints, where k 1max = The upper limit of the reactive current proportional coefficient of the hydropower low voltage ride through standard, so the reactive current proportional coefficient of low voltage ride through section 1 and low voltage ride through section 2 is k1=min{k 11 , k 1max}.

[0075] Low voltage ride through section three k2∈[max{k 2min , k min}, k 22min ], the hydropower unit maintains k1=k 1max The reactive current is increased while meeting the low voltage ride-through requirements, taking into account the output active power to the greatest extent possible, and the voltage at the grid connection point is restored after the fault ends.

[0076] If k 2min >k min This means that simply relying on the hydropower generator grid-side converter to increase active current cannot meet the low voltage ride-through requirements. An additional reactive power compensation device is needed to output a certain amount of reactive power to ensure that the grid support capacity requirements of the unit are met.

[0077] S3. Determine the coordinated control method of the grid-side converter, unloading circuit, and crowbar according to each voltage range.

[0078] When the voltage at the grid connection point drops, the speed regulator of the variable-speed hydropower unit reduces the guide vane opening according to the speed command, controls the flow of water entering the turbine runner, and reduces the output power of the turbine. The deviation Δω between the guide vane opening of the speed regulator of the variable-speed hydropower unit and the mechanical speed of the rotor m The transfer function between is expressed as:

[0079]

[0080] Where G(s) is the transfer function, y is the guide vane opening of the variable speed hydropower unit governor, T y is the response time of the main relay, K p , K i and K d are proportional, integral, and differential coefficients, respectively, T d is the differential attenuation constant, Δω m is the rotor mechanical speed deviation.

[0081] However, since the guide vane opening is a mechanical quantity, its adjustment process involves the opening and closing of mechanical devices and changes in water flow, and the adjustment time takes tens or even hundreds of seconds. Therefore, during the low voltage ride-through process, the variable-speed hydropower unit still needs to rely on the coordinated control of the grid-side converter, unloading circuit and crowbar to achieve rapid response and power balance in order to successfully carry out low voltage ride-through.

[0082] In the first low voltage ride-through stage, the grid-side converter outputs the active current before the fault, and at the same time, k1=min{k 11 , k 1max The proportional coefficient of} increases the reactive current and gives full play to the reactive support capability of the grid-side converter. The crowbar does not need to be put into operation, and the current of the rotor-side converter will not reach the limit. The unloading circuit is put into operation when the DC bus voltage is greater than or equal to the DC bus voltage limit to consume surplus power and protect the DC bus.

[0083] In the second low voltage ride-through stage, the grid-side converter outputs the active current before the fault, and at the same time, k1=min{k 11 , k 1max The proportional coefficient of} increases the reactive current and gives full play to the reactive support capability of the grid-side converter. The crowbar needs to be put into use to prevent overcurrent on the rotor side. The unloading circuit is put into use when the DC bus voltage is greater than or equal to the DC bus voltage limit to consume surplus power and protect the DC bus.

[0084] In the third stage of low voltage ride-through, the grid-side converter reduces the output active current and sets k1=k 1min The proportional coefficient increases reactive current to meet the grid-side converter current constraint and low voltage ride-through requirements. The crowbar is activated to prevent rotor-side overcurrent. The unloading circuit activates when the DC bus voltage is greater than or equal to the DC bus voltage limit, dissipating excess power and protecting the DC bus. In this embodiment, the DC bus voltage limit is 1.1 pu.

[0085] Example 2, reference Figure 3 and Figure 4 , which is the second embodiment of the present invention, provides a segmented control method for low voltage ride-through of a variable-speed hydropower unit. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through experiments.

[0086] The fault is set to a three-phase short circuit on the transmission line, with the voltage dropping to 0.6 pu. The fault occurs at 8 seconds and lasts for 0.5 seconds. Calculations based on system parameters indicate that the grid connection point voltage is in low voltage ride-through stage 1 at this time, with no active current reduction and no need for crowbar insertion. Figure 3 and Figure 4 are the output power of the hydropower unit and the DC bus voltage, respectively. It can be seen that at the moment the voltage at the grid connection point drops, the hydropower unit immediately increases the reactive power with k1=2.6 to support the voltage at the grid connection point. At the same time, the unloading circuit is put into use to consume the surplus power on the DC bus, and the DC bus voltage is maintained at 1.1pu during the fault. After the fault is over, the hydropower unit gradually recovers the active power and reactive power before the fault. Compared with the non-segmented low voltage ride-through control strategy that emits reactive power with k1=1.5, the method provided by the present invention can increase the reactive power emitted by the variable-speed hydropower unit and enhance the hydropower unit's ability to support the grid voltage.

[0087] Example 3, reference Figure 5 and Figure 6 , which is the third embodiment of the present invention, provides a segmented control method for low voltage ride-through of a variable-speed hydropower unit. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through experiments.

[0088] The fault is set to a three-phase short circuit on the transmission line, with the voltage dropping to 0.5 pu. The fault occurs at 8 seconds and lasts for 0.5 seconds. Calculations based on system parameters indicate that the grid connection point voltage is in low voltage ride-through stage 2 at this time. Active current remains unchanged, but a crowbar is required to protect the rotor-side converter. Figure 5 and Figure 6 are the output power of the hydropower unit and the DC bus voltage, respectively. It can be seen that at the moment the voltage at the grid connection point drops, the hydropower unit immediately increases the reactive power at k1=1.95 to support the voltage at the grid connection point. At the same time, the unloading circuit is put into use to consume the surplus power on the DC bus, and the DC bus voltage is maintained at 1.1pu during the fault. After the fault is over, the hydropower unit gradually recovers the active power and reactive power before the fault. Compared with the non-segmented low voltage ride-through control strategy that emits reactive power at k1=1.5, the method provided by the present invention can increase the reactive power emitted by the variable-speed hydropower unit and enhance the hydropower unit's ability to support the grid voltage.

[0089] Example 4, with reference toFigure 7 and Figure 8 This is the fourth embodiment of the present invention, which provides a segmented control method for low voltage ride-through of a variable-speed hydropower unit. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through experiments.

[0090] The fault is set to a three-phase short circuit on the high-voltage side of the sending transformer, causing the voltage to drop to 0.2 pu. The fault occurs at 8 seconds and lasts for 0.5 seconds. Calculations based on system parameters indicate that the grid connection point voltage is in low voltage ride-through (LVRT) level 3 at this time, generating only reactive current and no active current, necessitating the use of a crowbar. Figure 7 and Figure 8 The values ​​for (a) and (b) are the output power and DC bus voltage of the hydropower unit, respectively. A crowbar is installed on the rotor side to prevent the rotor-side converter current from exceeding the limit. A load shedding circuit is installed to dissipate excess power on the DC bus, maintaining the DC bus voltage at 1.1 pu during the fault. This shows that the hydropower unit can successfully complete low-voltage ride-through even when the fault is deep.

[0091] Example 5 is the fifth embodiment of the present invention, which differs from the first two embodiments in that:

[0092] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk.

[0093] The logic and / or steps represented in the flowcharts or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing the logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (e.g., a computer-based system, a system including a processor, or other system that can fetch and execute instructions from an instruction execution system, apparatus, or device). For purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by, or in conjunction with, an instruction execution system, apparatus, or device.

[0094] More specific examples (a non-exhaustive list) of computer-readable media include the following: an electrical connection with one or more wires (electronic devices), a portable computer disk cartridge (magnetic devices), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), a fiber optic device, and a portable compact disc read-only memory (CDROM). In addition, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering, or processing in another suitable manner as necessary, and then stored in a computer memory.

[0095] It should be understood that various parts of the present invention can be implemented using hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0096] Example 6, the sixth embodiment of the present invention, provides a segmented control system for low voltage ride-through of a variable-speed hydropower unit, including a variable reactive current proportional coefficient module and a collaborative control module.

[0097] The reactive current proportional coefficient variable module is configured to establish a reactive current proportional coefficient expression based on the fault voltage range, taking into account constraints such as low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault, and the converter current limit.

[0098] The collaborative control module is configured to establish collaborative issues between each converter and additional hardware; the converter output is determined by the variable reactive power proportional coefficient module, whether the crowbar is engaged is determined by the calculated voltage range, and the unloading circuit operating range is determined according to the DC bus voltage limit.

[0099] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A segmented control method for low voltage ride-through of a variable-speed hydropower unit, characterized by: include, Based on the detected voltage at the grid connection point, the fault voltage range is divided into three sections, taking into account whether the grid-side converter output active power remains unchanged during the low voltage ride-through period and whether the crowbar circuit is activated. According to the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault and the current limit constraints of the converter, the calculation expression of the reactive current proportional coefficient in each voltage range is determined; According to each voltage range, the coordinated control method of the grid-side converter, unloading circuit and crowbar is determined.

2. The method for controlling low voltage ride-through of a variable-speed hydroelectric generator set according to claim 1, characterized in that: The fault voltage interval is divided into three sections including low voltage ride through section 1, low voltage ride through section 2 and low voltage ride through section 3; Low voltage ride-through section 1 includes k2∈(k 21max , k max ], where k2 is the per-unit voltage of the hydropower unit grid connection point, k 21max is the voltage of the crowbar input, k max It is the upper limit of the standard voltage for low voltage ride through of hydropower units; The rotor-side converter is protected by a crowbar to avoid overcurrent under different outputs. Assuming that the generator is fully powered before the fault, the crowbar input voltage k 21max Expressed as, T s ′=L s ′R s T r ′=L r ′R r Among them, I rmax is the maximum current on the rotor side, I RSC is the current limit value of the rotor-side converter, L m is the mutual inductance between the stator and rotor in the dq coordinate system, L s is the stator self-inductance in the dq coordinate system, ω s is the synchronous speed, L r ′ is the rotor equivalent inductance, L r is the rotor self-inductance in the dq coordinate system, ψ r is the rotor flux amplitude, T is the system period, T s ′ is the time decay constant of the DC component of the stator flux, T r ′ is the time decay constant of the DC component of the rotor flux, L s ′ is the stator equivalent inductance, R s is the stator resistance, R r is the rotor resistance; Crowbar input voltage k 21max The meaning is that when the grid voltage is higher than k 21max When the doubly-fed induction motor is fully powered, the maximum value of the rotor current after the fault does not exceed the current limit of the rotor-side converter, and the crowbar does not need to be put into operation.

3. The method for controlling low voltage ride-through of a variable-speed hydroelectric generator set according to claim 2, wherein: The low voltage ride-through section 2 includes k2∈(k 22min , k 21max ], where k 22min To consider the grid-side converter current constraint, low voltage ride-through requires k1 ≥ k 1min Compared with the working condition before the fault I gd =I gd0+ The critical voltage during the post-low voltage ride-through period is expressed as, Among them, I gmax is the per-unit value of the AC side output current limit of the grid-side converter, I gd0+ is the per-unit value of the AC side active current output by the grid-side converter before the fault, I gd is the active current output by the AC side of the grid-side converter, k 1min The lower limit of the reactive current proportional coefficient of the hydropower low voltage ride-through standard; Critical voltage k 22min The meaning is the lowest voltage that the hydropower unit can ride through while keeping the grid-side converter output active current unchanged, under the premise of meeting the grid-side converter current constraint and low voltage ride-through requirements; Substitute each parameter into the calculation to get k 21max >k 22min .

4. The method for controlling low voltage ride-through of a variable-speed hydroelectric generator set according to claim 3, characterized in that: The low voltage ride-through section three includes k2∈[max{k 2min , k min }, k 22min ], where k min The lower limit of the standard voltage for low voltage ride through of hydropower units, k 2min To consider the low voltage ride through requirement k1 ≥ k 1min and the grid-side converter current constraint ΔI gq ≤I gmax , the limit voltage during low voltage ride-through is expressed as, Among them, ΔI gq The unit value of reactive current generated by the hydropower unit; Limit voltage k 2min The meaning of is to reduce the grid-side converter output active current and the minimum voltage that the hydropower unit rides through, while meeting the grid-side converter current constraint and low voltage ride-through requirements; The voltage interval is divided according to whether the active power output of the grid-side converter remains unchanged during the low voltage ride-through period and whether the crowbar circuit is activated. The fault voltage interval during the low voltage ride-through period is divided into three sections, which are expressed as: Among them, Crowbar is a crowbar.

5. The method for controlling low voltage ride-through of a variable-speed hydroelectric generator set according to claim 4, characterized in that: The reactive current proportional coefficient calculation expression for each voltage interval is determined based on the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault, and the current limit constraint of the converter, including: k2∈(k 22min , k max ], considering the grid-side converter current limit and the operating conditions of the hydropower unit before the fault, the reactive current proportional coefficient is expressed as, Among them, k 11 is the value of the reactive current proportional coefficient under the grid-side converter current constraint; In order to meet the low voltage ride through requirement of k 1min ≤k1≤k 1max Constraints, where k 1max = The upper limit of the reactive current proportional coefficient of the hydropower low voltage ride through standard, so the reactive current proportional coefficient of low voltage ride through section 1 and low voltage ride through section 2 is k1=min{k 11 , k 1max }; Low voltage ride through section three k2∈[max{k 2min , k min }, k 22min ], the hydropower unit maintains k1=k 1max The reactive current is increased while meeting the low voltage ride-through requirements, maximizing the output of active power. The voltage at the grid connection point is restored after the fault is resolved. If k 2min >k min This means that simply relying on the hydropower generator grid-side converter to increase active current cannot meet the low voltage ride-through requirements. An additional reactive power compensation device is needed to output a certain amount of reactive power to ensure that the grid support capacity requirements of the unit are met.

6. The method for controlling low voltage ride-through of a variable-speed hydroelectric generator set according to claim 4, characterized in that: The method of determining the coordinated control of the grid-side converter, the unloading circuit and the crowbar according to each voltage interval includes: When the voltage at the grid connection point drops, the speed regulator of the variable-speed hydropower unit reduces the guide vane opening according to the speed command, controls the flow of water entering the turbine runner, and reduces the output power of the turbine. The deviation Δω between the guide vane opening of the speed regulator of the variable-speed hydropower unit and the mechanical speed of the rotor m The transfer function between is expressed as, Where G(s) is the transfer function, y is the guide vane opening of the variable speed hydropower unit governor, T y is the response time of the main relay, K p , K i and K d are proportional, integral, and differential coefficients, respectively, T d is the differential attenuation constant, Δω m is the rotor mechanical speed deviation; During the low voltage ride-through process of variable-speed hydropower units, it is necessary to rely on the coordinated control of the grid-side converter, unloading circuit and crowbar to achieve rapid response and power balance to successfully carry out low voltage ride-through.

7. The method for controlling low voltage ride-through of a variable speed hydroelectric generator set according to claim 4, characterized in that: The method for determining the coordinated control of the grid-side converter, the unloading circuit and the crowbar according to each voltage interval also includes: In the first low voltage ride-through stage, the grid-side converter outputs the active current before the fault, and at the same time, k1=min{k 11 , k 1max The proportional coefficient of the current regulator increases the reactive current, giving full play to the reactive support capability of the grid-side converter. The crowbar does not need to be engaged, and the current of the rotor-side converter will not reach the limit. The unloading circuit is engaged when the DC bus voltage is greater than or equal to the DC bus voltage limit, consuming surplus power and protecting the DC bus. In the second low voltage ride-through stage, the grid-side converter outputs the active current before the fault, and at the same time, k1=min{k 11 , k 1max The proportional coefficient of the current is used to increase the reactive current and give full play to the reactive support capability of the grid-side converter. The crowbar needs to be activated to prevent overcurrent on the rotor side. The unloading circuit is activated when the DC bus voltage is greater than or equal to the DC bus voltage limit to consume surplus power and protect the DC bus. In the third stage of low voltage ride-through, the grid-side converter reduces the output active current and sets k1=k 1min The proportional coefficient increases the reactive current to meet the grid-side converter current constraint and low voltage ride-through requirements. The crowbar needs to be activated to prevent overcurrent on the rotor side. The unloading circuit is activated when the DC bus voltage is greater than or equal to the DC bus voltage limit to consume surplus power and protect the DC bus.

8. A variable speed hydroelectric generator low voltage ride through segmented control system, using a variable speed hydroelectric generator low voltage ride through segmented control method according to any one of claims 1 to 7, characterized in that: include: Variable reactive current proportional coefficient module and collaborative control module; The reactive current proportional coefficient variable module is configured to establish a reactive current proportional coefficient expression based on the fault voltage range, taking into account the low voltage ride-through requirements, the operating conditions of the hydropower unit before the fault, and the converter current limit constraint conditions; The collaborative control module is configured to establish a collaborative problem between each converter and additional hardware; wherein the converter output is determined by the variable reactive power proportional coefficient module, whether the crowbar is engaged is determined by the calculated voltage range, and the unloading circuit working range is determined according to the DC bus voltage limit.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the low voltage ride-through segmented control method of a variable speed hydropower unit according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of a segmented control method for low voltage ride-through of a variable speed hydropower unit according to any one of claims 1 to 7 are implemented.

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