Flexible DC power transmission system voltage oscillation risk assessment method based on Lyapunov direct method

Abstract

The invention discloses a Lyapunov direct method-based voltage oscillation risk assessment method for a flexible DC power transmission system, and the method comprises the following steps: 1, obtaining a droop control parameter and a rated voltage grade of the flexible DC power transmission system, the transmission power of the flexible direct-current power transmission system is acquired through a wide-area measurement system, and the direct-current voltage at the outlet of the converter station is controlled by adopting constant power; 2, calculating a system balance point and related stability indexes; 3, judging whether the direct-current voltage of the system is unstable or not by using a Lyapunov direct method; and 4, outputting a risk level result according to a stability judgment result. Compared with an existing risk assessment method, the method is suitable for assessing the transient DC voltage oscillation stability of the flexible DC power transmission system, and is high in practicability and good in economical efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible DC power transmission, and relates to the technical field of power system security and stability analysis and risk assessment. Specifically, it relates to a method for risk assessment of voltage oscillation in a flexible DC power transmission system based on Lyapunov's direct method. Background Art

[0002] There are deep - seated dynamic behaviors and interactive coupling phenomena between the converter and the power grid, which easily lead to the oscillation problem of DC voltage and thus limit the power transmission capacity of the DC power transmission system. In recent years, different degrees of DC oscillation have occurred during the commissioning process of multiple flexible DC power transmission projects at home and abroad, which has seriously affected the power quality and safe and stable operation of the new energy transmission system in desert - gobi areas. In addition, most of the existing methods rely on artificial intelligence. While they cannot clearly explain the instability mechanism of DC voltage oscillation, they also consume a large amount of computing resources and time. Therefore, studying how to use the stability mechanism to quickly assess the risk of DC voltage transient oscillation instability has very important engineering significance. Summary of the Invention

[0003] In order to solve the problems existing in the above - mentioned prior art, the purpose of the present invention is to propose a method for risk assessment of voltage oscillation in a flexible DC power transmission system based on Lyapunov's direct method, which directly uses the stability mechanism to quickly assess the risk of DC voltage transient oscillation instability without relying on artificial intelligence algorithms.

[0004] To achieve the above - mentioned purpose, the present invention adopts the following technical solutions:

[0005] A method for risk assessment of voltage oscillation in a flexible DC power transmission system based on Lyapunov's direct method, which combines the equilibrium point of the flexible DC power transmission system and two major stability indicators, and uses Lyapunov's direct method to achieve a quick assessment of the risk of DC voltage transient oscillation instability, including the following steps:

[0006] Step 1: Obtain the droop control parameters and rated voltage level of the flexible DC power transmission system, and collect the transmission power of the flexible DC power transmission system through a wide - area measurement system and measure the DC voltage at the outlet of the converter station with constant power and control;

[0007] Step 2: Calculate the equilibrium point and related stability indicators of the flexible DC power transmission system

[0008] According to the collected transmission power of the flexible DC power transmission system and the parameters of the flexible DC power transmission system itself, calculate the equilibrium point (U rec(s) , I c(s) ) of the flexible DC power transmission system after being disturbed, and two stability indicators U rec(1) and U rec(2) , and the specific expressions are as follows:

[0009]

[0010] Wherein, R e and L e are respectively the equivalent resistance and equivalent inductance of the DC transmission line; C dc is the DC capacitor at the DC outlet side; K D is the droop control parameter; U dcn is the rated DC voltage; P t is the measured value of the transmission power of the flexible DC transmission system; Δ is the discriminant of the roots of the quadratic equation;

[0011] Step 3: Determine the DC voltage transient oscillation stability by using the Lyapunov direct method

[0012] Considering that after the flexible DC transmission system is disturbed, the relationship between the equilibrium point of the flexible DC transmission system and the two major stability indicators will change. The following stability criterion is obtained by using the Lyapunov direct method and the limit cycle theory to determine the DC voltage transient oscillation stability of the flexible DC transmission system;

[0013] U rec(r) > min{U rec(1) , U rec(2)}, stable

[0014] U rec(s) < min{U rec(1) , U rec(2)} unstable

[0015] Wherein, U rec(r) is the real-time measured value of the DC voltage at the outlet of the constant power control converter station;

[0016] In addition, considering the conservativeness of the Lyapunov direct method, when the system operating state does not meet the above stability criterion, a medium-risk result is output to remind the operator to pay attention at all times;

[0017] Step 4: Output the risk assessment result according to the stability discrimination result

[0018] The hierarchical output of the risk assessment result is realized according to the discrimination result of the DC voltage transient oscillation stability of the flexible DC transmission system in Step 3.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] The method of the present invention can evaluate the risk of DC voltage transient oscillation instability by calculating the balance point and two major stability indexes of the VSC-HVDC system, and use the Lyapunov direct method to evaluate the risk without artificial intelligence methods. It can quickly evaluate the transient operation risk of the flexible DC transmission system, with strong feasibility and good economy. It realizes the hierarchical early warning of the DC voltage safety of the Shagehuang new energy transmitted through the VSC-HVDC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the flowchart of the present invention;

[0022] Figure 2 is the topology diagram of the Shagehuang new energy base transmitted through the VSC-HVDC system in the present invention;

[0023] Figure 3 is the DC voltage response waveform of the embodiment under different power mutations;

[0024] Figure 4 is the phase plane diagram of the DC voltage and capacitor current of the embodiment under the same power mutation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The present invention will be further described in detail below with reference to the drawings and embodiments.

[0026] As Figure 1 shown, the embodiment of the present invention provides a method for evaluating the voltage oscillation risk of a VSC-HVDC system based on the Lyapunov direct method. The specific steps are as follows:

[0027] Step 1: Obtain the droop control parameter K D of the VSC-HVDC system and the rated voltage level, and collect the transmission power P t of the VSC-HVDC system through the wide area measurement system, and adopt constant power and control the DC voltage U dcn at the outlet of the converter station;

[0028] Step 2: Calculate the balance point and related stability indexes of the VSC-HVDC system

[0029] According to the rated transmission power of the collected VSC-HVDC system and the self-parameters of the VSC-HVDC system, that is, the droop control parameter K D , the balance point (U rec(s) , I c(s) ) of the VSC-HVDC system after being disturbed and two stability indexes U rec(1) and U rec(2) can be calculated. The specific expressions are as follows:

[0030]

[0031] In the formula, R e and Le are the equivalent resistance and equivalent inductance of the DC transmission line respectively; C dc is the DC capacitor on the DC outlet side; K D is the droop control parameter; U dcn is the rated DC voltage; P t is the measured value of the transmission power of the flexible DC transmission system; Δ is the discriminant of the roots of the quadratic equation of one variable.

[0032] Step 3: Determine the DC voltage transient oscillation stability using the Lyapunov direct method

[0033] Considering that after the flexible DC transmission system is disturbed, the relationship between the equilibrium point of the flexible DC transmission system and the two major stability indicators will change. The following stability criterion can be obtained using the Lyapunov direct method and the limit cycle theory to determine the DC voltage transient oscillation stability of the flexible DC transmission system.

[0034] U rec(r) > min{U rec(1) , U rec(2)}, stable (low risk)

[0035] U rec(s) < min{U rec(1) , U rec(2)}, unstable (high risk)

[0036] In the formula, U rec(r) is the real-time measured value of the DC voltage at the outlet of the constant power control converter station.

[0037] In addition, considering the conservativeness of the Lyapunov direct method, when the system operating state does not meet the above stability criterion, a medium-risk result is output to remind the operator to pay attention at all times.

[0038] Step 4: Output the risk assessment result according to the stability discrimination result

[0039] Realize the hierarchical output of the risk assessment result based on the discrimination result of the DC voltage transient oscillation stability of the flexible DC transmission system in Step 3.

[0040] As Figure 1 shown, if the determination result is stable, a low risk is output; if the determination result is unstable, a high risk is output; if the determination result does not satisfy the inequality relationship in Step 3, a medium risk is output.

[0041] Embodiment:

[0042] The method of the present invention is for the risk assessment of the DC voltage transient oscillation instability of the flexible DC transmission system.

[0043] In order to verify the correctness of the proposed assessment method, a simulation platform is built asFigure 2 The shown flexible DC power transmission system model uses the flexible DC external power transmission system from the Shagehuang new energy base.

[0044] The simulation verification was carried out according to the following parameters.

[0045] The rated voltage of the DC system is 100 kV; the rated capacity of the system is 100 MVA; the converter station is set to unity power factor control, and the initial transmission power is set to 20 MW; the droop control parameter is set to 0.

[0046] As Figure 3 shown in (a) below, when the power mutation is small, the DC voltage of the flexible DC power transmission system is oscillating and stable. At this time, the minimum DC voltage (97.46 kV) of the flexible DC power transmission system after being disturbed is greater than the proposed stability criteria (63.95 kV & 79.77 kV), and the output result is low risk, verifying the correctness of the stability criterion; as Figure 3 shown in (b) below, when the power mutation gradually increases, the oscillation amplitude of the DC voltage of the flexible DC power transmission system gradually increases. At this time, the proposed stability criterion cannot distinguish the DC voltage oscillation stability of the flexible DC power transmission system, and the output result is medium risk to remind relevant operation and maintenance personnel to pay attention to the system operation trend; as Figure 3 shown in (c) below, as the power mutation further increases, an unstable limit cycle will appear in the phase plane of the flexible DC power transmission system and finally an aperiodic instability phenomenon will occur. At this time, the equilibrium point (98.77 kV) of the DC voltage of the flexible DC power transmission system is much smaller than the two major stability discrimination criteria (126.13 kV & 161.07 kV), and the output result is high risk, verifying the correctness of the unstable discrimination criterion proposed in the present invention.

[0047] As Figure 4 shown in (a) below, when the power mutation is small, the phase plane diagram of the flexible DC power transmission system is stable. The system operating point quickly approaches the equilibrium point after being disturbed, and the outermost end of the phase plane trajectory is always on the right side of the stability criterion, and the output result is low risk, intuitively verifying the correctness of the proposed stability criterion; as Figure 4 shown in (b), when the power mutation gradually increases, the phase plane diagram of the flexible DC power transmission system is still stable, but the outermost end of the phase plane trajectory no longer satisfies the proposed stability criterion, intuitively showing the conservatism of the Lyapunov direct method. Therefore, the output result is medium risk to remind relevant operation and maintenance personnel to pay attention to the system operation trend; as Figure 4 shown in (c) below, as the power mutation further increases, the phase plane diagram of the flexible DC power transmission system is unstable, and the outermost end of the phase plane trajectory and the two major stability criteria satisfy the quantitative relationship of the proposed unstable criterion, and the output result is high risk, intuitively verifying the correctness of the unstable criterion proposed in the present invention.

[0048] In summary, the present invention relates to a method for evaluating the voltage oscillation risk of a flexible DC transmission system based on the Lyapunov direct method. By combining the equilibrium point of the flexible DC transmission system and two major stability indicators, the Lyapunov direct method is used to evaluate the risk of DC voltage transient oscillation instability. Compared with the existing artificial intelligence methods, the present invention can quickly evaluate the transient operation risk of the flexible DC transmission system, and has strong practicability and good economy.

Claims

1. A voltage oscillation risk assessment method for a flexible DC transmission system based on the Lyapunov direct method, characterized in that: Combined with the equilibrium point and two major stability indicators of the flexible DC transmission system, the Lyapunov direct method is used to achieve a rapid assessment of the risk of DC voltage transient oscillation instability, including the following steps: Step 1: Obtain the droop control parameters and rated voltage level of the flexible DC transmission system, and collect the transmission power of the flexible DC transmission system through the wide-area measurement system and measure the DC voltage at the outlet of the constant power and control converter station; Step 2: Calculate the equilibrium point and related stability indicators of the flexible DC transmission system According to the transmitted power of the flexible DC transmission system collected and the parameters of the flexible DC transmission system itself, calculate the equilibrium point (U rec(s) , I c(s) ) of the flexible DC transmission system after being disturbed, as well as two stability indices U rec(1) and U rec(2) . The specific expressions are as follows: where R e and L e are the equivalent resistance and equivalent inductance of the DC transmission line respectively; C dc is the DC capacitor on the DC outlet side; K D is the droop control parameter; U dcn is the rated DC voltage; P t is the measured value of the transmission power of the flexible DC transmission system; Δ is the discriminant of the roots of the quadratic equation; Step 3: Determine the DC voltage transient oscillation stability using the Lyapunov direct method Considering that after the flexible DC transmission system is disturbed, the relationship between the equilibrium point and the two major stability indicators of the flexible DC transmission system will change, the following stability criterion is obtained using the Lyapunov direct method and the limit cycle theory to determine the DC voltage transient oscillation stability of the flexible DC transmission system; U rec(r) > min{U rec(1) , U rec(2)}, stable U rec(s) <min{U rec(1) ,U rec(2)} is unstable Wherein, U rec(r) is the real-time measured value of the DC voltage at the outlet of the constant power control converter station; In addition, considering the conservativeness of the Lyapunov direct method, when the system operating state does not meet the above stability criterion, a medium-risk result is output to remind the operator to pay attention at all times; Step 4: Output the risk assessment result according to the stability discrimination result Based on the discrimination result of the DC voltage transient oscillation stability of the flexible DC transmission system in Step 3, a hierarchical output of the risk assessment result is realized.

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