AC / DC system sending end transient overvoltage suppression method based on improved VDCOL curve

Abstract

The invention belongs to the field of power system transient stability, and provides an AC / DC system sending end transient overvoltage suppression method based on an improved VDCOL curve. Comprising the following steps: 1) constructing an LCC-HVDC system of which a sending end system comprises a new energy unit; obtaining the bus power consumption of each node of the LCC-HVDC system, and carrying out the normalization processing; 2) substituting the obtained electrical quantity into a transient overvoltage expression, and evaluating whether the voltage of each node exceeds a transient overvoltage discrimination index; the method comprises the following steps: deducing a sending-end bus transient overvoltage expression of the LCC-HVDC system according to a short-circuit ratio; constructing a transient overvoltage evaluation index according to the expression; and judging whether the LCC-HVDC system generates transient overvoltage or not according to the evaluation index, and suppressing the transient overvoltage at a sending-end bus by switching a control curve of a VDCOL link to realize the stability of system operation.

Description

Technical Field

[0001] The invention belongs to the field of transient stability of power systems and proposes a method for suppressing transient overvoltage at the sending end of an AC or DC system based on an improved VDCOL curve. Background Art

[0002] To improve the uneven distribution of energy and the high transmission losses, my country has adopted a hybrid high-voltage AC / DC transmission method for power transmission. In recent years, cross-regional transmission technology for renewable energy has made further progress. High-voltage DC transmission systems can achieve large-capacity, long-distance power transmission, and hybrid AC / DC transmission has become an important means of transmitting renewable energy. This paper first analyzes the causes and influencing factors of transient overvoltages at the wind turbine connection point when an AC fault occurs in the receiving system, considering the connection of wind turbines to the sending-end grid. By using the variation in reactive power compensation, a calculation expression for transient overvoltages under the interaction of AC / DC systems is established, and an evaluation index for transient overvoltages is further derived. Secondly, a method for suppressing transient overvoltages at the sending end of AC / DC systems based on an improved VDCOL curve is proposed. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, which is scientific, reasonable, efficient and practical.

[0004] A method for suppressing transient overvoltage at the sending end of an AC / DC system by improving the VDCOL curve includes the following contents:

[0005] Clarify the generation mechanism of transient overvoltage

[0006] Commutation failure causes a bypass on the inverter side, shorting the DC side and rapidly increasing DC current. This leads to a rapid increase in rectifier reactive power consumption, which in turn draws a significant amount of reactive power from the sending AC system, causing the sending AC grid voltage to drop. Under the action of the low-voltage current limiting control link (VDCOL), the trigger angle on the rectifier side increases rapidly, causing the DC current to decrease rapidly, even dropping to zero. At this point, the reactive power consumed by the rectifier decreases rapidly, but the rectifier station's AC filter continues to operate, resulting in a reactive power surplus network at the rectifier station. This feeds a large amount of reactive power into the sending AC system, causing transient overvoltages in the sending AC grid.

[0007] Constructing a transient overvoltage assessment index based on short-circuit ratio

[0008] In the case of the same power system network topology, the parameters of each component in the system are different, and the corresponding short-circuit ratio SCR is also different:

[0009]

[0010] Where Q dN is the rated reactive power of the DC system; S c is the commutation busbar short-circuit capacity, which can be expressed as:

[0011]

[0012] Where X and U N are the equivalent reactance and equivalent potential of the transmitting AC system respectively.

[0013] Based on the equivalence of AC and DC short-circuit capacity and stable power transmission to the AC system, and considering the residual reactive capacity of the converter station and the compensation of transient voltage rise, the transient voltage at the rectifier bus is derived:

[0014]

[0015] Based on the transient voltage expression, the transient overvoltage evaluation index T is derived. r for

[0016]

[0017] Indicator T r It is approximately proportional to the voltage value after the busbar fault, that is, T r The larger it is, the greater the transient overvoltage amplitude generated by the bus at the sending end.

[0018] Transient overvoltage suppression method using improved VDCOL curve

[0019] During the boost process, the rectifier side is usually in a constant current control state under the action of VDCOL. The DC current limit value is obtained based on the current DC voltage, and then compared with the current reference value to output the current command and adjust the trigger angle of the rectifier side. In the standard DC test system CIGRE, the functional relationship of the VDCOL curve is:

[0020]

[0021] Where: I dmin 、U dL and U dH Set to 0.55, 0.4 and 0.9; conventional three-stage VDCOL control optimization usually only adjusts the inflection point parameters of the curve, and the control flexibility of the intermediate transition section is often not high.

[0022] This paper uses the Tanh function to obtain an improved VDCOL control curve as shown below:

[0023]

[0024] where Ψ H With Ψ LThe analytical expression is as follows:

[0025]

[0026]

[0027] Tanh function can well limit the value range to [I dmin Within the range of [1], the smooth nature of the function facilitates connection with the straight lines on either side, facilitating a smooth transition from transient to steady-state control. Furthermore, by adding only two parameters, the Tanh function can achieve not only the standard "S" curve but also other requirements for constructing different curve forms. In some cases, the function can also be easily converted into a similar form to a traditional broken line, making it easy to obtain the initial parameters of the improved function and coordinate the transition with traditional control methods, thus showing high applicability.

[0028] The above-mentioned design scheme achieves the following beneficial effects: First, an expression for transient overvoltage on the sending-end busbar of an LCC-HVDC system is derived based on the short-circuit ratio; a transient overvoltage evaluation index is constructed based on this expression; and based on the evaluation index, whether the LCC-HVDC system generates transient overvoltage is determined. By switching the control curve of the VDCOL link, transient overvoltage at the sending-end busbar is suppressed. Finally, a simulation of an LCC-HVDC system incorporating wind turbines into the sending-end system demonstrates that switching the VDCOL link to the improved control curve effectively suppresses transient overvoltages and maintains system stability, thus verifying the effectiveness of the proposed method. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0030] Figure 1 This is a framework diagram of an LCC-HVDC system containing new energy units, based on an improved VDCOL curve and a method for suppressing transient overvoltage at the sending end of an AC / DC system.

[0031] Figure 2 This is a curve showing the relationship between evaluation index and transient voltage of a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to the present invention;

[0032] Figure 3 A comparison diagram of an improved curve of a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to the present invention and a traditional curve;

[0033] Figure 4 This is a transient voltage curve of a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to the present invention;

[0034] Figure 5This is a reactive power curve of a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to the present invention;

[0035] Figure 6 The present invention discloses a DC current curve for a method of suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve. DETAILED DESCRIPTION

[0036] The present invention provides a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve. Figures 1 to 6 , including the following steps:

[0037] 1. Constructing an LCC-HVDC system with a new energy generator set at the sending end. Figure 1 The following description will be given using the system structure diagram as an example.

[0038] 2. Construct transient overvoltage expression

[0039] In the case of the same power system network topology, the parameters of each component in the system are different, and the corresponding short-circuit ratio SCR is also different:

[0040]

[0041] Where QdN is the rated reactive power of the DC system; Sc is the commutation bus short-circuit capacity, which can be expressed as:

[0042]

[0043] Where X and UN are the equivalent reactance and equivalent potential of the transmitting AC system, respectively.

[0044] Assuming that the line impedance is ignored, it can be seen from the power flow calculation formula that the bus voltage change at the sending end is:

[0045]

[0046] When a fault occurs in the LCC-HVDC system, the total reactive power compensation on the sending bus is Qc, and the voltage of the sending bus after the fault is U1a. After the fault occurs, the voltage of the sending bus increases, and the reactive power compensation capacity Qca also increases accordingly:

[0047] When a fault occurs, the bus voltage at the sending end rises, and ΔU1 is as follows:

[0048]

[0049] Based on the equivalence of AC and DC short-circuit capacity and stable power transmission to the AC system, and considering the residual reactive capacity of the converter station and the compensation of transient voltage rise, the transient voltage at the rectifier bus is derived:

[0050]

[0051] 3. Construct transient overvoltage assessment indicators

[0052] As can be seen from the above formula, Qca decreases with voltage after a fault. The busbar reactive compensation device is divided into capacitor Qcl and AC filter Qf. Therefore, the total reactive compensation capacity Qc on the sending-end busbar is expressed as:

[0053]

[0054] For the convenience of calculation, let

[0055]

[0056] Then there exists an expression:

[0057]

[0058] The above equation is a quartic equation. The Ferriera root formula method is used to solve the sending end bus voltage during a fault, as shown in the following equation:

[0059]

[0060]

[0061] The indicator Tr is approximately proportional to the voltage value after the bus fault, that is, the larger the Tr is, the greater the transient overvoltage amplitude generated by the bus at the sending end.

[0062] 4. VDCOL control curve after design improvement

[0063] 1) Tanh function

[0064]

[0065] The hyperbolic tangent function is used to design the control curve of the VDCOL link.

[0066] 2) Improved VDCOL link function curve

[0067]

[0068] The analytical expressions of ΨH and ΨL are as follows:

[0069]

[0070]

[0071] Where Ud is the transient voltage at the sending-end bus; UdL and UdH are the set minimum and maximum transient voltage values, respectively; Id is the DC voltage; and Idmin is the set minimum DC current value. By adjusting the values ​​of β1 and β2, the shape of the curve can be adjusted to better suppress transient overvoltages.

[0072] 5. Model calculation process

[0073] ① First, build an LCC-HVDC system with new energy units at the sending end.

[0074] ② Obtain the bus power consumption of each node in the LCC-HVDC system through WAMS (Wide Area Measurement System) and perform normalization.

[0075] ③Substitute the obtained electrical quantity into the transient overvoltage expression to evaluate whether the voltage of each node exceeds the transient overvoltage judgment index.

[0076] ④ When the voltage of a certain node is higher than the identification index, the control curve of the VDCOL link is switched to the designed improved curve, and the transient overvoltage is suppressed by using the control curve based on the improved tanh function.

[0077] In-depth analysis Figures 3 to 6 The experimental results show that:

[0078] 1) By Figures 3-4 As can be seen, the VDCOL curve improved by the tanh function is steeper than the traditional curve, reducing the system's active power deficit during the system voltage recovery phase and facilitating rapid DC current recovery. When the system voltage returns to near normal, the VDCOL curve is designed to be flatter to ensure a smooth transition from transient to steady state and reduce system interaction power fluctuations. When a transient overvoltage occurs at the sending-end bus, switching the VDCOL control curve from the traditional curve to the improved curve significantly reduces the transient voltage at the bus. This demonstrates that the improved VDCOL curve can effectively suppress transient overvoltages and improve system voltage stability.

[0079] 2) By Figures 5-6 It can be seen that the reactive power and DC current curves at the sending-end bus also change depending on the VDCOL control curve. During a fault, when the sending-end bus voltage drops, reactive power flows in the opposite direction; when a transient overvoltage occurs, reactive power flows back from the rectifier side to the DC line. After the fault occurs, constant current control is performed on the rectifier side, and the improved curve control in the VDCOL link significantly reduces reactive power consumption at the commutation bus. Because the improved curve consumes less active power during voltage recovery, the DC current recovers faster. The constant current controller also reduces the transient voltage amplitude increase, thus suppressing transient overvoltage.

[0080] The calculation conditions, legends, etc. in the embodiments of the present invention are only used to further illustrate the present invention and are not exhaustive and do not constitute a limitation on the scope of protection of the claims. Those skilled in the art can conceive of other substantially equivalent alternatives based on the inspiration gained from the examples of the present invention without creative work, and all of them are within the scope of protection of the present invention.

Claims

1. A method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, characterized in that: The following steps are involved: 1) Construct an LCC-HVDC system with a new energy generating unit in the sending end system; obtain the bus power consumption of each node in the LCC-HVDC system and perform normalization processing; 2) Substitute the obtained electrical quantity into the transient overvoltage expression and evaluate whether the voltage of each node exceeds the transient overvoltage judgment index; 3) When the voltage of a certain node is higher than the identification index, the control curve of the VDCOL link is switched to the designed improved curve, and the transient overvoltage is suppressed by using the control curve based on the improved tanh function.

2. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 1, characterized in that: The transient overvoltage expressions include: Under the same network topology of the power system, the short circuit ratio S CR The expression is as follows: Where Q dN is the rated reactive power of the DC system; S c is the commutation busbar short-circuit capacity, which can be expressed as: Where X and U N are the equivalent reactance and equivalent potential of the transmitting AC system respectively; The bus voltage change at the sending end is: When a fault occurs in the LCC-HVDC system, the total reactive power compensation on the sending end bus is Q c , after the fault, the bus voltage at the sending end is U 1a After the fault occurs, the sending end bus voltage increases, and the reactive compensation capacity Q ca It also increases: When a fault occurs, the bus voltage at the sending end rises, and ΔU1 is transformed into: Based on the equivalence of AC and DC short-circuit capacity and stable power transmission to the AC system, and considering the residual reactive capacity of the converter station and transient voltage rise compensation, the transient voltage at the rectifier bus is derived:

3. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 2, characterized in that: Evaluation of whether the voltage of each node exceeds the transient overvoltage judgment index includes: Q after the fault ca As the voltage decreases; the busbar reactive compensation device is divided into capacitor Q cl and AC filter Q f , so the total reactive power compensation capacity Q on the sending end bus c Expressed as: For the convenience of calculation, let Then there exists an expression: The above equation is a quartic equation. The Ferriera root formula method is used to solve the sending end bus voltage during a fault, as shown in the following equation: Indicator T r It is approximately proportional to the voltage value after the busbar fault, that is, T r The larger it is, the greater the transient overvoltage amplitude generated by the bus at the sending end.

4. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 3, characterized in that: The improved VDCOL link control curve includes: 1) Tanh function The hyperbolic tangent function is used to design the control curve of the VDCOL link; 2) Improved VDCOL link function curve where Ψ H With Ψ L The analytical expression is as follows: Where U d is the transient voltage at the sending end bus; U dL 、U dH are the set minimum and maximum values ​​of transient voltage respectively; I d is the DC voltage; I dmin is the set minimum value of DC current; by adjusting the values ​​of β1 and β2, the shape of the curve can be adjusted to better suppress transient overvoltage.

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