Flexible excitation system transient reactive power characteristic modeling method under power grid voltage fault

Abstract

The invention discloses a transient reactive characteristic modeling method for a flexible excitation system under a power grid voltage fault, and the structure and a control block diagram of the system meet the following requirements: a synchronous generator is connected with a three-phase voltage source type converter, the three-phase voltage source type converter is connected with a chopping converter, the chopping converter outputs excitation voltage according to a system control signal, and the excitation voltage is converted into the excitation voltage. Therefore, excitation current is provided for the synchronous machine. The modeling process comprises the following steps: analyzing a synchronous machine equivalent diagram and an AVR control block diagram in the flexible excitation system so as to establish a corresponding mathematical model; then voltage and current parameters of a synchronous machine end of the system in a steady state before a fault are calculated; the transient current caused by the stator voltage change and the excitation voltage change is further solved respectively; and finally, the transient reactive power of the synchronous machine side of the flexible excitation system is obtained. The method provides a theoretical basis for realizing dynamic strong excitation control of the flexible excitation system and improving the stability of the system under a transient condition.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly to a modeling method for transient reactive power characteristics of a flexible excitation system under grid voltage faults. Background Art

[0002] With the wide application of UHV AC / DC technologies, the connection of a large number of new energy units to the grid, and the commissioning of large-scale energy storage systems, the overall power grid in China presents two major characteristics: UHV AC / DC hybrid operation and power electronics. However, long-distance power transmission not only causes significant power losses but also makes long transmission lines vulnerable to faults, resulting in a reduced stability margin of the receiving-end power system, thus greatly weakening the local transient voltage support ability and making the transient voltage stability problem increasingly severe. This situation of long-distance power transmission, high receiving-end load, and insufficient local power sources has gradually highlighted the transient voltage stability problem of the receiving-end system and led to voltage collapse, thereby increasing the risk of local load shedding or large-scale power outages.

[0003] Generator excitation control is generally regarded as one of the most economical and efficient power system control technologies in the industry because it requires no additional equipment and has strong system stability regulation capabilities. Traditional thyristor-based excitation systems use thyristor rectifiers as power units, and their excitation voltage output ability is relatively dependent on the terminal voltage of the generator. When the terminal voltage drops significantly, it is difficult to maintain sufficient generator field forcing ability. Summary of the Invention

[0004] The purpose of the present invention is to provide a modeling method for transient reactive power characteristics of a flexible excitation system under grid voltage faults. This method takes into account the influence of grid voltage fault conditions on the transient reactive power characteristics of synchronous machines, providing a theoretical basis for the dynamic field forcing control of flexible excitation systems to improve system transient stability.

[0005] To achieve the above object, the present invention provides the following technical solution: A modeling method for transient reactive power characteristics of a flexible excitation system under grid voltage faults, where the topological structure of the flexible excitation system and the control block diagram of the excitation control system respectively meet the following conditions: A synchronous generator is connected to a three-phase voltage source converter, the three-phase voltage source converter is connected to a chopper converter, and the chopper converter outputs an excitation voltage according to the control signal of the system to provide excitation for the synchronous machine; The method includes the following steps:

[0006] S1, establish a transient and steady-state model of the flexible excitation system;

[0007] S2, analyze the topological structure of the flexible excitation system and the control block diagram of the excitation control system, determine the equivalent circuit diagram and dynamic mathematical model of the synchronous generator, and determine its control method through analysis of the control block diagram of the excitation control system;

[0008] S3. Calculate the voltage and current of the flexible-excitation synchronous machine under steady state before the fault: Calculate the terminal voltage and current of the machine under steady state before the fault according to the equivalent circuit diagram of the flexible-excitation synchronous machine and the power of the synchronous machine before the fault occurs.

[0009] S4. Calculate the transient current caused only by the change in stator voltage: Calculate the transient current on the machine side of the synchronous machine caused only by the change in stator voltage according to the equivalent circuit diagram of the flexible-excitation synchronous machine and the change in stator voltage under the fault.

[0010] S5. Calculate the transient current caused only by the change in excitation voltage: Calculate the transient current on the machine side of the synchronous machine caused only by the change in excitation voltage according to the control method and control parameters of the excitation control.

[0011] S6. Calculate the transient reactive power on the machine side of the flexible-excitation synchronous machine: Calculate the transient reactive power output on the machine side of the flexible-excitation synchronous machine under the power grid voltage fault according to the voltage and current of the flexible-excitation synchronous machine under steady state before the fault, the change in stator voltage, and the transient current caused by the change in excitation voltage.

[0012] Furthermore, in S2, the control part of the excitation control system includes an integrator and a limiter. The function of the limiter is to output the excitation voltage E f .

[0013] Among them, the transfer function of the first integrator is

[0014]

[0015] Among them, K A is the proportional amplification factor of the excitation system, T A is the time constant of the power amplifier, and s is the integration operator.

[0016] Among them, the transfer function of the first integrator is

[0017]

[0018] Among them, T B , T C are the time constants of the power unit stability loop, and s is the integration operator.

[0019] Among them, the limiter is used to obtain the voltage signal within the range of [V RMIN , V RMAX , where V RMIN is the minimum excitation voltage, and V RMAX is the maximum excitation voltage.

[0020] Furthermore, the stator voltage equation of the synchronous generator is:

[0021]

[0022] Furthermore, transform the stator voltage equation from the three-phase coordinate system to the dq two-phase rotating coordinate system:

[0023]

[0024] Furthermore, the rotor voltage equation of the synchronous generator is:

[0025]

[0026] The flux linkage equation in the dq two-phase rotating coordinate system is:

[0027]

[0028] Furthermore, obtain the fifth-order dynamic mathematical model of the synchronous motor:

[0029]

[0030] Furthermore, calculate the voltage and current of the flexible-excitation synchronous machine under the pre-fault steady state:

[0031]

[0032] Among them, the subscript 0 represents the components of the voltage and current under the pre-fault steady state, u sd , u sq , i sd and i sq are the components of the stator voltage and current on the dq axes respectively. U base is the amplitude of the grid voltage phase voltage at steady state, and I base is the amplitude of the phase current output to the grid at steady state.

[0033] Furthermore, the transient current caused only by the change in the stator voltage is expressed in the complex frequency domain as:

[0034]

[0035] Among them, the subscript 1 represents the transient component generated only by the action of the stator voltage, u 1sd is the d-axis component of the machine-side voltage when only the stator voltage acts, u 1sq is the q-axis component of the machine-side voltage when only the stator voltage acts, i 1sd is the d-axis component of the machine-side current when only the stator voltage acts, i 1sq is the q-axis component of the machine-side current when only the stator voltage acts, I 1f is the field current of the machine-side when only the stator voltage acts, i 1rd is the d-axis component of the rotor-side current when only the stator voltage acts, i 1rqThe q-axis component of the rotor-side current when only the stator voltage acts, R s is the stator winding resistance, L sd is the d-axis leakage inductance, L sq is the q-axis leakage inductance, L md is the d-axis mutual inductance, L mq is the q-axis mutual inductance, R f is the field winding resistance, L f is the field winding inductance, R rd is the rotor d-axis resistance, R rq is the rotor q-axis resistance, L rd is the rotor d-axis inductance, L rq is the rotor q-axis inductance, ω is the angular frequency of the synchronous machine, and s is the integral operator.

[0036] By applying the inverse Laplace transform to the current in the complex frequency domain, the transient current caused by the change in the stator voltage in the time domain can be obtained.

[0037] Furthermore, the transient current caused by the change in the field voltage is expressed in the complex frequency domain as:

[0038]

[0039] where the subscript 2 represents the transient component generated by only the field voltage, U 2f is the voltage on the machine side when only the field voltage acts, i 2sd is the d-axis component of the current on the machine side when only the field voltage acts, i 2sq is the q-axis component of the current on the machine side when only the field voltage acts, I 2f is the field current on the machine side when only the field voltage acts, i 2rd is the d-axis component of the rotor-side current when only the field voltage acts, i 2rq is the q-axis component of the rotor-side current when only the field voltage acts, R s is the stator winding resistance, L sd is the d-axis leakage inductance, L sq is the q-axis leakage inductance, L md is the d-axis mutual inductance, L mq is the q-axis mutual inductance, R f is the field winding resistance, L f is the field winding inductance, R rd is the rotor d-axis resistance, R rq is the rotor q-axis resistance, L rd is the rotor d-axis inductance, L rq is the rotor q-axis inductance, ω is the angular frequency of the synchronous machine, and s is the integral operator.

[0040] By applying the inverse Laplace transform to the current signal in the complex frequency domain, the transient current caused by the change in the field voltage in the time domain can be obtained.

[0041] The transient voltage and current equations of the flexible excitation synchronous machine obtained by superposition are as follows:

[0042]

[0043] Furthermore, the calculation equation of the transient reactive power of the synchronous machine is: Q s (t) = 1.5u sd (t)i sq (t)

[0044] Where: Q s is the transient reactive power output on the machine side of the synchronous generator, u 0sd is the d-axis component of the machine-side voltage under steady state before the fault, u 1sd is the d-axis component of the machine-side voltage when only the stator voltage acts, u 0sq is the d-axis component of the grid voltage under steady state before the fault, u 1sq is the q-axis component of the voltage when only the stator voltage acts, I 0sd is the d-axis component of the machine-side current under steady state before the fault, i 1sd is the d-axis component of the machine-side current when only the stator voltage acts, i 2sd is the d-axis component of the machine-side current when only the excitation voltage acts, i 0sq is the q-axis component of the machine-side current under steady state before the fault, i 1sq is the q-axis component of the machine-side current when only the stator voltage acts, i 2sq is the q-axis component of the machine-side current when only the excitation voltage acts. Description of the Drawings

[0045] Figure 1 is the equivalent circuit diagram of the synchronous generator under the d-axis and q-axis; among them, Figure 1(a) is the equivalent circuit diagram of the synchronous generator under the d-axis; Figure 1(b) is the equivalent circuit diagram of the synchronous generator under the q-axis.

[0046] Figure 2 is the control block diagram of the flexible excitation system.

[0047] Figure 3 is the simulation model of the flexible excitation system.

[0048] Figure 4 is the flowchart of a method for modeling the transient reactive power characteristics of a flexible excitation system under grid voltage faults of the present invention. Detailed Embodiments

[0049] In order to describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the drawings and specific implementation methods.

[0050] The new excitation system uses a rectifier based on fully controlled devices such as IGBT, IGCT, and GTO as the power unit. Therefore, it is called a fully controlled device excitation system, and due to its flexible control, it is also called a flexible excitation system. During large disturbance faults such as generator terminal short circuits in the power system, the flexible excitation system, due to the use of a converter based on fully controlled devices, has good boosting ability and can still ensure the strong excitation ability of the excitation system. Therefore, it is of great theoretical and practical significance to calculate the transient reactive power characteristics on the machine side of the flexible excitation system under grid faults.

[0051] The present invention provides a method for modeling the transient reactive power characteristics of a flexible excitation system under grid voltage faults. As shown in Figures 1 to Figure 4 shown, the topological structure of the flexible excitation system and the control block diagram of the excitation control system respectively meet the following conditions: The synchronous generator is connected to a three-phase voltage source converter, the three-phase voltage source converter is connected to a chopper converter, and the chopper converter outputs the excitation voltage according to the control signal of the system to provide excitation for the synchronous machine. The method includes the following steps:

[0052] S1, establish the transient and steady-state models of the flexible excitation system;

[0053] S2, analyze the topological structure of the flexible excitation system and the control block diagram of the excitation control system, determine the equivalent circuit diagram and dynamic mathematical model of the synchronous generator, and determine its control method through the analysis of the control block diagram of the excitation control system;

[0054] S3, calculate the voltage and current of the flexible excitation synchronous machine under pre-fault steady state: According to the equivalent circuit diagram of the flexible excitation synchronous machine and the power of the synchronous machine before the fault occurs, calculate the voltage and current on the machine side under pre-fault steady state;

[0055] S4, calculate the transient current caused only by the change in stator voltage: According to the equivalent circuit diagram of the flexible excitation synchronous machine and the change in stator voltage under the fault, calculate the transient current on the machine side of the synchronous machine caused only by the change in stator voltage;

[0056] S5, calculate the transient current caused only by the change in excitation voltage: According to the control method and control parameters of the excitation control, calculate the transient current on the machine side of the synchronous machine caused only by the change in excitation voltage;

[0057] S6, calculate the transient reactive power on the machine side of the flexible excitation synchronous machine: According to the voltage and current of the flexible excitation synchronous machine under pre-fault steady state, the transient current caused by the change in stator voltage and the change in excitation voltage, calculate the transient reactive power output on the machine side of the flexible excitation synchronous machine under grid voltage faults.

[0058] Figure 1 shows the equivalent circuit diagrams under the d-axis and q-axis of a synchronous generator, as shown in Figures 1(a) and 1(b). According to the equivalent diagrams, the fifth-order dynamic mathematical model of the synchronous motor can be derived. Then, by using the superposition theorem of power sources, the transient response calculation of the flexible excitation system under grid voltage faults is divided into three state components: the steady state before the fault (state 0), the state with only stator voltage change (state 1), and the state with only excitation voltage change (state 2). In Figure 1, L l : leakage inductance of the stator winding, L fld , L lfd : leakage inductance of the excitation winding, L lkd : leakage inductance of the damper winding, R kd : resistance of the damper winding, R fd : resistance of the excitation winding, L md , L mq : mutual inductance, L lkq1 , L lkq2 : leakage inductance of the damper winding, R kq1 , R kq2 : resistance of the damper winding. Figure 2 where K A is the proportional amplification factor of the excitation system, T A is the time constant of the power amplifier, and s is the integral operator. T B , T C are the time constants of the stable loop of the power unit, and s is the integral operator. V RMIN is the minimum excitation voltage, V RMAX is the maximum excitation voltage. V t is the generator terminal voltage, V t * is the reference value of the generator terminal voltage.

[0059] In the embodiment of the present invention, S3 is specifically:

[0060] The following equations are used to calculate the pre-fault steady-state voltage and current:

[0061]

[0062] where: the subscript 0 represents the components of the voltage and current in the pre-fault steady state, u sd , u sq , i sd and i sq are the components of the stator voltage and current on the dq axes respectively; U base is the amplitude of the grid voltage phase voltage in the steady state, and I base is the amplitude of the phase current output to the grid in the steady state.

[0063] In the embodiment of the present invention, S4 is specifically:

[0064] The following equations are used to calculate the voltage and current on the machine side of the synchronous machine when only the stator voltage changes:

[0065]

[0066] Among them, the subscript 1 represents the transient component generated by the action of only the stator voltage, u 1sd is the d-axis component of the voltage on the machine side when only the stator voltage acts, u 1sq is the q-axis component of the voltage on the machine side when only the stator voltage acts, i 1sd is the d-axis component of the current on the machine side when only the stator voltage acts, i 1sq is the q-axis component of the current on the machine side when only the stator voltage acts, I 1f is the excitation current on the machine side when only the stator voltage acts, i 1rd is the d-axis component of the current on the rotor side when only the stator voltage acts, i 1rq is the q-axis component of the current on the rotor side when only the stator voltage acts, R s is the stator winding resistance, L sd is the d-axis leakage inductance, L sq is the q-axis leakage inductance, L md is the d-axis mutual inductance, L mq is the q-axis mutual inductance, R f is the excitation winding resistance, L f is the excitation winding inductance, R rd is the rotor d-axis resistance, R rq is the rotor q-axis resistance, L rd is the rotor d-axis inductance, L rq is the rotor q-axis inductance, ω is the angular frequency of the synchronous machine, and s is the integral operator.

[0067] In the embodiment of the present invention, S5 is specifically:

[0068] The following equations are used to calculate the voltage and current on the machine side of the synchronous machine when only the excitation voltage changes:

[0069]

[0070] Among them, the subscript 2 represents the transient component generated by the action of only the excitation voltage, U 2f is the voltage on the machine side when only the excitation voltage acts, i 2sd is the d-axis component of the current on the machine side when only the excitation voltage acts, i 2sq is the q-axis component of the current on the machine side when only the excitation voltage acts, I 2f is the excitation current on the machine side when only the excitation voltage acts, i 2rd is the d-axis component of the current on the rotor side when only the excitation voltage acts, i 2rq is the q-axis component of the current on the rotor side when only the excitation voltage acts, R s is the stator winding resistance, Lsd is the d-axis leakage inductance, L sq is the q-axis leakage inductance, L md is the d-axis mutual inductance, L mq is the q-axis mutual inductance, R f is the resistance of the field winding, L f is the inductance of the field winding, R rd is the rotor d-axis resistance, R rq is the rotor q-axis resistance, L rd is the rotor d-axis inductance, L rq is the rotor q-axis inductance, ω is the angular frequency of the synchronous machine, and s is the integral operator.

[0071] In the embodiment of the present invention, S6 is specifically:

[0072] The transient voltage and current equations of the flexible-excitation synchronous machine are calculated using the following equations:

[0073]

[0074] The calculation equation for the transient reactive power of the synchronous machine is: Q s (t) = 1.5u sd (t)i sq (t);

[0075] Where: Q s is the transient reactive power output from the machine side of the synchronous generator, u 0sd is the d-axis component of the machine-side voltage under pre-fault steady state, u 1sd is the d-axis component of the machine-side voltage when only the stator voltage acts, u 0sq is the d-axis component of the grid voltage under pre-fault steady state, u 1sq is the q-axis component of the voltage when only the stator voltage acts, I 0sd is the d-axis component of the machine-side current under pre-fault steady state, i 1sd is the d-axis component of the machine-side current when only the stator voltage acts, i 2sd is the d-axis component of the machine-side current when only the field voltage acts, i 0sq is the q-axis component of the machine-side current under pre-fault steady state, i 1sq is the q-axis component of the machine-side current when only the stator voltage acts, i 2sq is the q-axis component of the machine-side current when only the field voltage acts.

Claims

1. A transient reactive power characteristic modeling method for a flexible excitation system under grid voltage faults, characterized in that The topological structure of the flexible excitation system and the control block diagram of the excitation control system respectively meet the following conditions: The synchronous generator is directly connected to the three-phase voltage source converter, and the three-phase voltage source converter is connected to the chopper converter, which outputs the excitation voltage according to the system control signal to provide excitation for the synchronous machine; The method includes the following steps: S1. Establish a transient and steady-state model of the flexible excitation system; S2. Analyze the topological structure of the flexible excitation system and the control block diagram of the excitation control system, determine the equivalent circuit diagram and dynamic mathematical model of the synchronous generator, and determine its control method through the analysis of the control block diagram of the excitation control system; S3. Calculate the voltage and current of the flexible excitation synchronous machine under pre-fault steady state: According to the equivalent circuit diagram of the flexible excitation synchronous machine and the power of the synchronous machine before the fault occurs, calculate the voltage and current on the machine side under pre-fault steady state; S4. Calculate the transient current caused only by the change in stator voltage: According to the equivalent circuit diagram of the flexible excitation synchronous machine and the change in stator voltage under the fault, calculate the transient current on the machine side of the synchronous machine caused only by the change in stator voltage; S5. Calculate the transient current caused only by the change in excitation voltage: According to the control method and control parameters of the excitation control, calculate the transient current on the machine side of the synchronous machine caused only by the change in excitation voltage; S6. Calculate the transient reactive power on the machine side of the flexible excitation synchronous machine: According to the voltage and current of the flexible excitation synchronous machine under pre-fault steady state, the transient current caused by the change in stator voltage and the change in excitation voltage, calculate the transient reactive power output on the machine side of the flexible excitation synchronous machine under the grid voltage fault.

2. The transient reactive power characteristic modeling method of the flexible excitation system under grid voltage faults according to claim 1, characterized in that The specific content of S3 is as follows: Use the following equations to calculate the pre-fault steady-state voltage and current: where: the subscript 0 represents the component of the voltage and current in the pre-fault steady state, u sd , u sq , i sd and i sq are the components of the stator voltage and current on the dq axes respectively; U base is the amplitude of the grid voltage phase voltage in the steady state, and I base is the amplitude of the phase current output to the grid in the steady state.

3. The transient reactive power characteristic modeling method of the flexible excitation system under grid voltage faults according to claim 1, characterized in that, The specific content of S4 is as follows: The transient current caused only by the change in stator voltage is expressed in the complex frequency domain as: Where: subscript 1 represents the transient component generated by the stator voltage alone, u 1sd is the d-axis component of the terminal voltage when only the stator voltage acts, u 1sq is the q-axis component of the terminal voltage when only the stator voltage acts, i 1sd is the d-axis component of the terminal current when only the stator voltage acts, i 1sq is the q-axis component of the terminal current when only the stator voltage acts, I 1f is the field current of the terminal when only the stator voltage acts, i 1rd is the d-axis component of the rotor-side current when only the stator voltage acts, i 1rq is the q-axis component of the rotor-side current when only the stator voltage acts, R s is the stator winding resistance, L sd is the d-axis leakage inductance, L sq is the q-axis leakage inductance, L md is the d-axis mutual inductance, L mq is the q-axis mutual inductance, R f is the field winding resistance, L f is the field winding inductance, R rd is the rotor d-axis resistance, R rq is the rotor q-axis resistance, L rd is the rotor d-axis inductance, L rq is the rotor q-axis inductance, ω is the angular frequency of the synchronous machine, and s is the integral operator.

4. The transient reactive power characteristic modeling method of the flexible excitation system under grid voltage faults according to claim 1, characterized in that The specific content of S5 is as follows: The transient current caused only by the change in excitation voltage is expressed in the complex frequency domain as: where: the subscript 2 represents the transient component generated by the action of the exciting voltage only, and U 2f is the voltage on the machine side when only the exciting voltage acts, and i 2sd is the d-axis component of the current on the machine side when only the exciting voltage acts, and i 2sq is the q-axis component of the current on the machine side when only the exciting voltage acts, and I 2f is the exciting current on the machine side when only the exciting voltage acts, and i 2rd is the d-axis component of the current on the rotor side when only the exciting voltage acts, and i 2rq is the q-axis component of the current on the rotor side when only the exciting voltage acts, and R s is the stator winding resistance, and L sd is the d-axis leakage inductance, and L sq is the q-axis leakage inductance, and L md is the d-axis mutual inductance, and L mq is the q-axis mutual inductance, and R f is the exciting winding resistance, and L f is the exciting winding inductance, and R rd is the rotor d-axis resistance, and R rq is the rotor q-axis resistance, and L rd is the rotor d-axis inductance, and L rq is the rotor q-axis inductance, ω is the angular frequency of the synchronous machine, and s is the integral operator; By applying the inverse Laplace transform to the current signal in the complex frequency domain, the transient current caused by the change in excitation voltage in the time domain can be obtained.

5. The transient reactive power characteristic modeling method of the flexible excitation system under grid voltage faults according to claim 1, characterized in that The specific content of S6 is as follows: Use the following calculation equations to solve the transient reactive power on the machine side of the synchronous machine: The transient reactive power output by the machine side of the flexible excitation synchronous machine under the grid voltage fault is calculated, and its calculation equation is: Q s (t) = 1.5u sd (t)i sq (t); Where: Q s is the transient reactive power output from the machine side of the synchronous generator, u 0sd is the d-axis component of the machine-side voltage under pre-fault steady state, u 1sd is the d-axis component of the machine-side voltage when only the stator voltage acts, u 0sq is the d-axis component of the grid voltage under pre-fault steady state, u 1sq is the q-axis component of the voltage when only the stator voltage acts, I 0sd is the d-axis component of the machine-side current under pre-fault steady state, i 1sd is the d-axis component of the machine-side current when only the stator voltage acts, i 2sd is the d-axis component of the machine-side current when only the excitation voltage acts, i 0sq is the q-axis component of the machine-side current under pre-fault steady state, i 1sq is the q-axis component of the machine-side current when only the stator voltage acts, i 2sq is the q-axis component of the machine-side current when only the excitation voltage acts.

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