CN111416375B - Commutation failure prediction method considering transient direct current change process - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention discloses a commutation failure prediction method considering a transient direct current change process, which comprises the following steps: (1) obtaining original parameters required by calculation; (2) Calculating a forefront trigger angle and transient direct current of the inversion side system after the fault occurs according to the existing parameters; (3) calculating the actual off angle of the system at 3.3ms after the fault occurs; (4) Predicting commutation failure by comparing with an inherent limit off angle; the method can overcome the defect that the existing HVDC system commutation failure voltage prediction criterion is not accurate enough, avoid the adverse consequences of direct current transmission power fluctuation and the like caused by misoperation or refusal of a commutation failure defense strategy due to misjudgment or omission, and has a certain practical value for safe and stable operation of a large-capacity long-distance alternating current-direct current interconnected power grid.
Description
Technical Field
The invention relates to a high-voltage direct-current transmission technology, in particular to a commutation failure prediction method considering a transient direct-current change process.
Background
Due to special geographical environment conditions and regional development characteristics, the east and west of China are greatly unbalanced in terms of energy production and consumption. Implementing the "western electric east send" energy strategy is an important measure to solve this problem. Twenty or more conventional dc transmission projects have been built and put into operation nationwide since the eighties of the twentieth century. Compared with alternating current transmission, high-voltage direct current transmission has unique advantages in the aspects of asynchronous networking, short-circuit capacity control, rapid power adjustment, transmission distance, line loss and the like, has become an important ring in the energy strategy of China, greatly promotes national economic development and improvement of the living standard of people, and generates huge economic benefit and social benefit. However, conventional HVDC engineering also has some problems, and commutation failure is one of them. Failure to commutate refers to failure to recover blocking capability for a period of time after the valve is out of operation, and to re-open after the valve voltage transitions to the forward direction. Once commutation failure occurs, the adverse consequences of overlarge direct current, reduced direct current transmission power, relay protection misoperation, direct current magnetic bias of the converter transformer, shortened service life of a converter valve, unstable voltage of a weak alternating current system at the inversion side and the like are caused. If the control is improper, continuous commutation failure can be caused, even the direct current transmission power is interrupted, the power flow is transferred in a large range, and the stability problem of the interconnected power grid is caused. Therefore, the influence factors of commutation failure are deeply studied, a feasible prevention and control strategy is formed, and the method has important significance for reducing the probability of commutation failure and ensuring the safe and stable operation of the HVDC system and the interconnected power grid. The current HVDC system adopts a commutation failure inhibition strategy, which mainly adopts the modes of sending trigger pulse in advance after detecting that an AC system fails, controlling DC current and the like to control the HVDC system. The starting criterion is to collect three-phase voltage of a bus of a convertor station through a voltage transformer, process the three-phase voltage and compare the three-phase voltage with a starting threshold value to judge whether to control the system. In other words, the voltage drop of the commutation bus caused by the ac fault at the inversion side of the HVDC system is taken as the basis for judging whether the commutation failure occurs. However, in principle, the critical voltage drop of the commutation failure of the HVDC system is affected by a plurality of factors, and if only a fixed threshold value is used as the commutation failure prediction criterion to be inaccurate in some occasions, erroneous judgment or missed judgment may occur, so that the malfunction or refusal of the commutation failure defense strategy is caused, and the stable operation or power recovery of the HVDC system is affected.
Disclosure of Invention
The technical problems to be solved by the invention are as follows: based on principle analysis of commutation failure, a commutation failure voltage prediction criterion considering the running state of the system before failure and current dynamics after failure is provided. The method can accurately predict whether the HVDC system has commutation failure when the AC fault occurs, overcomes the defect that the commutation failure is caused by inaccurate existing criteria, prevents control strategy malfunction or refusal, and has certain value for reducing the fluctuation of the transmission power of the HVDC system, maintaining the stable operation of the HVDC system and improving the safety of the AC/DC interconnected power grid.
In order to achieve the aim of the invention, the technical scheme adopted by the invention is as follows: an improved HVDC system commutation failure voltage prediction criterion is characterized in that: considering the running state of the HVDC system before the fault, decomposing the equivalent circuit of the HVDC system by utilizing the superposition principle, fitting the change process of the direct current after the three-phase alternating current fault occurs by a time sequence segmentation method, calculating the transient direct current at the corresponding moment according to the voltage of the commutation bus in the fault, and calculating the actual off angle in the fault process according to the transient direct current. And according to the action and turn-off angle change rule of the HVDC control system, comparing the actual turn-off angle of 3.3ms after the fault occurs with the inherent limit turn-off angle of the thyristor to predict whether the commutation failure occurs.
Based on the definition, the commutation failure prediction method considering the transient direct current change process comprises the following steps:
(1) Obtaining original parameters required by calculation
Obtaining tuning parameters (I) of HVDC system before AC fault d And γ) and other steady state operating parameters; acquiring inverter-side converter bus alternating voltage E of 3.3ms after alternating current fault occurs if 。
(2) Calculating the forefront trigger angle and transient DC current of the inversion side system after the fault occurs according to the existing parameters
According to the tuning parameters of the HVDC system before failure, the system touches more forwardHair angle beta 0 The calculation may be performed according to the following formula. Wherein X is c The commutation reactance representing the system can be calculated from system parameters and can be considered constant during analysis.
According to the alternating voltage of the commutation bus after the fault, the following time sequence segmentation method can be utilized to solve the formula to calculate the transient direct current I at the inversion side after the fault occurs df 。
Wherein L, C, R are inductance, capacitance and resistance parameters of the direct current line, omega, delta and tau are constants calculated by the parameters; u (U) dr ，U dif Rectifying the side direct current voltage and the inversion side direct current voltage after the fault occurs respectively.
(3) Calculating the actual turn-off angle of the system at 3.3ms after the fault occurs
Solving the actual closing angle gamma of the system of 3.3ms after the fault occurs according to the closing angle calculation formula of the HVDC system a
(4) Predicting commutation failure by comparing with inherent limit off angle
The calculated actual closing angle gamma a Inherent limit Guan Duanjiao gamma to thyristors min By comparison, if gamma a ＜γ min The ac fault is predicted to cause commutation failure. Otherwise, the prediction may be successfully commutated.
The time-series segment current calculation method is specifically described below.
(1) Superposition method for calculating transient direct current
The equivalent circuit of the HVDC system may be represented as a T-type equivalent circuit as shown in fig. 2. After three-phase fault occurs, the direct current at the inversion side consists of two parts, namely current flowing to the inversion side from the rectification side and current discharged to the inversion side from the line equivalent capacitance. According to the superposition principle of the linear circuit, the equivalent circuit of the HVDC system can be decomposed into a sub-circuit (1) and a sub-circuit (2) which are respectively solved, and then the sub-circuit and the sub-circuit are added to obtain the inversion side direct current in the fault process, as shown in figure 3.
By solving the sub-circuit (1) a current component I is obtained d1
Current component I which can be determined by means of a sub-circuit (2) d2
The transient DC current at the inversion side after the fault is generated is
Wherein L, C, R are inductance, capacitance and resistance parameters of the direct current line, omega, delta and tau are constants calculated by the parameters; u (U) dr ，U dif Rectifying the side direct current voltage and the inversion side direct current voltage after the fault occurs respectively.
(2) Time sequence segmentation method
If the alternating current fault occurrence time is taken as a time zero point, t is the time after the fault occurs f Is divided into N shorter time intervals, each time interval lasting Δt=t f N corresponds to 0, Δt,2Δt … … t f The voltage of the current converting bus at the moment is E respectively i (0),E i (Δt),E i (2Δt)……E i (t f ). Wherein the initial time voltage E i (0)＝E i End time voltage E i (t f )＝E if The voltages at each of the remaining intermediate moments can be approximated by means of one interpolation.
At the time delta t, the voltage of the converter bus is E i (Δt), the DC current adopts an initial value I d (0)＝I d The DC current I at the delta t time can be calculated according to the method d (Δt). The current is taken as an initial value of the current at the moment of 2 delta t to be combined with the voltage E of the converting bus at the moment of 2 delta t i (2Δt), the direct current … … at 2Δt time can be calculated, and so on, and t can be finally obtained by N times of calculation f Time of day direct current I d (t f )。
To ensure the accuracy of the result, two intermediate variables are required to be continuously corrected in the solving process to t s Time is an example (t s At a certain time, 0, Δt,2Δt … … (N-1) Δt, the specific correction formula is as follows.
U dr (t s )＝U dr0 cosα-I d (t s )d r ＝U dr (0)-(I d (t s )-I d (0))d r
Wherein omega 0 ，
Also calculated from L, C, R.
Accordingly, the iterative solving expression for solving the direct current in the fault process by the time sequence segmentation method can be summarized as
Advantageous effects
Based on the mechanism analysis of commutation failure, the invention analyzes transient change process of the HVDC system after three-phase AC fault occurrence to obtain theoretical expression of actual off angle of the system in fault, thereby carrying out commutation failure prediction. The method can overcome the defect that the existing HVDC system commutation failure voltage prediction criterion is not accurate enough, avoid the adverse consequences of direct current transmission power fluctuation and the like caused by misoperation or refusal of a commutation failure defense strategy due to misjudgment or omission, and has a certain practical value for safe and stable operation of a large-capacity long-distance alternating current-direct current interconnected power grid.
Drawings
FIG. 1 is a flow chart of a commutation failure prediction method that accounts for transient DC variation processes.
Fig. 2 is a schematic diagram of an HVDC system equivalent circuit.
Fig. 3 is an exploded schematic diagram of the HVDC system equivalent circuit.
Fig. 4 is a diagram showing a judgment result of the improved commutation failure voltage prediction criterion.
Detailed Description
The invention provides an improved HVDC system commutation failure voltage prediction criterion, which considers the running state of an HVDC system before a fault, decomposes an equivalent circuit of the HVDC system by utilizing a superposition principle, fits the change process of direct current after a three-phase alternating current fault occurs by a time sequence segmentation method, calculates transient direct current at a corresponding moment according to the voltage of a commutation bus in the fault, and calculates the actual break angle in the fault process according to the transient direct current. And according to the action and turn-off angle change rule of the HVDC control system, comparing the actual turn-off angle of 3.3ms after the fault occurs with the inherent limit turn-off angle of the thyristor to predict whether the commutation failure occurs.
Based on the definition, the commutation failure prediction method considering the transient direct current change process comprises the following steps:
(1) Obtaining original parameters required by calculation
Obtaining tuning parameters (I) of HVDC system before AC fault d And γ) and other steady state operating parameters; acquiring inverter-side converter bus alternating voltage E of 3.3ms after alternating current fault occurs if 。
(2) Calculating the forefront trigger angle and transient DC current of the inversion side system after the fault occurs according to the existing parameters
According to the tuning parameters of the HVDC system before failure, the system is more front in trigger angle beta 0 The calculation may be performed according to the following formula. Wherein X is c The commutation reactance representing the system can be calculated from system parameters and can be considered constant during analysis.
According to the alternating voltage of the commutation bus after the fault, the following time sequence segmentation method can be utilized to solve the formula to calculate the transient direct current I at the inversion side after the fault occurs df 。
Wherein L, C, R are inductance, capacitance and resistance parameters of the direct current line, omega, delta and tau are constants calculated by the parameters; u (U) dr ，U dif Rectifying the side direct current voltage and the inversion side direct current voltage after the fault occurs respectively.
(3) Calculating the actual turn-off angle of the system at 3.3ms after the fault occurs
Solving the actual closing angle gamma of the system of 3.3ms after the fault occurs according to the closing angle calculation formula of the HVDC system a
(4) Predicting commutation failure by comparing with inherent limit off angle
The calculated actual closing angle gamma a Inherent limit Guan Duanjiao gamma to thyristors min By comparison, if gamma a ＜γ min The ac fault is predicted to cause commutation failure. Otherwise, the prediction may be successfully commutated.
The time-series segment current calculation method is specifically described below.
(1) Superposition method for calculating transient direct current
The equivalent circuit of the HVDC system may be represented as a T-type equivalent circuit as shown in fig. 2. After three-phase fault occurs, the direct current at the inversion side consists of two parts, namely current flowing to the inversion side from the rectification side and current discharged to the inversion side from the line equivalent capacitance. According to the superposition principle of the linear circuit, the equivalent circuit of the HVDC system can be decomposed into a sub-circuit (1) and a sub-circuit (2) which are respectively solved, and then the sub-circuit and the sub-circuit are added to obtain the inversion side direct current in the fault process, as shown in figure 3.
By solving the sub-circuit (1) a current component I is obtained d1
Current component I which can be determined by means of a sub-circuit (2) d2
The transient DC current at the inversion side after the fault is generated is
Wherein L, C, R are inductance, capacitance and resistance parameters of the direct current line, omega, delta and tau are constants calculated by the parameters; u (U) dr ，U dif Rectifying the side direct current voltage and the inversion side direct current voltage after the fault occurs respectively.
(2) Time sequence segmentation method
If the alternating current fault occurrence time is taken as a time zero point, t is the time after the fault occurs f Is divided into N shorter time intervals, each time interval lasting Δt=t f N corresponds to 0, Δt,2Δt … … t f The voltage of the current converting bus at the moment is E respectively i (0),E i (Δt),E i (2Δt)……E i (t f ). Wherein, the initial timeVoltage E i (0)＝E i End time voltage E i (t f )＝E if The voltages at each of the remaining intermediate moments can be approximated by means of one interpolation.
At the time delta t, the voltage of the converter bus is E i (Δt), the DC current adopts an initial value I d (0)＝I d The DC current I at the delta t time can be calculated according to the method d (Δt). The current is taken as an initial value of the current at the moment of 2 delta t to be combined with the voltage E of the converting bus at the moment of 2 delta t i (2Δt), the direct current … … at 2Δt time can be calculated, and so on, and t can be finally obtained by N times of calculation f Time of day direct current I d (t f )。
To ensure the accuracy of the result, two intermediate variables are required to be continuously corrected in the solving process to t s Time is an example (t s At a certain time, 0, Δt,2Δt … … (N-1) Δt, the specific correction formula is as follows.
U dr (t s )＝U dr0 cosα-I d (t s )d r ＝U dr (0)-(I d (t s )-I d (0))d r
Wherein omega 0 ，
Also calculated from L, C, R.
Accordingly, the iterative solving expression for solving the direct current in the fault process by the time sequence segmentation method can be summarized as
Claims (3)
1. A commutation failure prediction method considering transient direct current change process is characterized in that:
(1) Acquiring original parameters required by calculation;
(2) Calculating a forefront trigger angle and transient direct current of the inversion side system after the fault occurs according to the existing parameters;
(3) Calculating an actual off angle of the system at 3.3ms after the fault occurs;
(4) Predicting commutation failure by comparing with an inherent limit off angle, wherein:
the transient direct current is based on the alternating voltage of the commutation bus after the fault, and the transient direct current I at the inversion side after the fault is generated can be calculated by utilizing the following time sequence segmentation method solving formula df ：
Wherein L, C, R are inductance, capacitance and resistance parameters of the direct current line, omega, delta and tau are constants calculated by the parameters; u (U) dr ，U dif Rectifying the side direct current voltage and the inversion side direct current voltage after the fault occurs respectively.
2. The commutation failure prediction method according to claim 1, wherein the commutation failure prediction method is based on transient dc current variation, and is characterized by: the front trigger angle of the inversion side system is based on the tuning parameters of the HVDC system before failure, and the front trigger angle beta of the system 0 The calculation can be made according to the following formula:
wherein X is c The commutation reactance representing the system can be calculated from system parameters and can be considered constant during analysis.
3. The seed meter of claim 1 and a transient dc current change processThe phase failure prediction method is characterized in that: the actual breaking angle is the actual breaking angle gamma of the system of 3.3ms after the fault is solved according to the breaking angle calculation formula of the HVDC system a
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