Method for evaluating stability improvement capability of controllable commutation current converter on power system

Abstract

The invention provides a method for evaluating the stability improvement capability of a power system for a controllable commutation converter (CLCC), so as to solve the problem of lack of quantitative evaluation on the stability effect of the novel CLCC in the prior art. Specifically, the method aims at comprehensively evaluating the effects of the CLCC in the aspects of suppressing commutation failure, relieving voltage fluctuation and enhancing the system anti-disturbance capability by defining an effective system strength ratio (SCESSR) of a single-circuit direct current system and an effective cooperative strength ratio (MCESSR) of a multi-circuit direct current system and combining simulation verification. Therefore, a scientific basis is provided for power grid planning, operation control and fault diagnosis, and the safety and stability of the AC / DC interconnected power grid in a new energy high-proportion scene are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current (HVDC) transmission, and in particular to a multi-DC coordinated control method in a HVDC transmission system and an ultra-high-voltage direct current (UHVDC) transmission system containing controllable phase-commutating valves. Background Art

[0002] To address the issue of continuous blocking in multi-infeed DC systems, fundamentally eliminate the long-term threat of commutation failures, and thus improve system stability, State Grid Corporation of China (SGCC) has collaborated with relevant research institutes and industrial entities to continuously improve the DC transmission system's AC fault ride-through capability and the AC system's reactive power and voltage support capabilities, reducing the risk of commutation failures. In 2020, they proposed the Controlled Commutated Converter (CLCC) solution. This converter leverages the high capacity and low losses of thyristors and the strong turn-off capability of IGBTs, leveraging their strengths to overcome their weaknesses. This solution inherits the economic advantages of traditional LCC converters while fundamentally avoiding commutation failures and improving grid stability. However, there is a lack of clear evaluation methods for the stability-enhancing capabilities of CLCCs in power systems. Summary of the Invention

[0003] This paper proposes a method for evaluating the ability of controlled-commutation converters (CLCCs) to improve power system stability, addressing the existing lack of quantitative evaluation of the stability effects of new CLCC converters. Specifically, this method aims to define the "Sensitive System Strength Ratio (SCESSR)" for single-circuit DC systems and the "Sensitive Coordinated Strength Ratio (MCESSR)" for multi-circuit DC systems. Combined with simulation verification, this method comprehensively evaluates the effectiveness of CLCCs in suppressing commutation failures, mitigating voltage fluctuations, and enhancing system anti-disturbance capabilities. This method provides a scientific basis for grid planning, operational control, and fault diagnosis, and improves the safety and stability of interconnected AC / DC power grids in scenarios with a high proportion of renewable energy.

[0004] The purpose of the present invention is achieved through the following technical solutions: A method for evaluating the ability of a controllable commutated converter to improve power system stability, the method comprising: S100. Determine whether the DC system in the power grid is a single-circuit DC system or a multi-circuit DC system; S200. Build a single-circuit DC power system model, propose an effective system strength ratio, and evaluate the stability improvement effect in the single-circuit DC system; S300. Build a multi-circuit DC power system model, propose an effective coordination intensity ratio, and evaluate the stability improvement effect of the multi-circuit DC system; S400. Use simulation tools to verify the accuracy of the stability improvement effect.

[0005] Preferably, S100 includes: Based on the topological feature analysis, the nodes of the converter stations are obtained. If there are only two converter stations in the power grid and the DC lines are point-to-point connected, it is determined to be a single-circuit DC system. If there are ≥3 converter stations or multiple groups of independent converter units are configured in the same converter station, it is determined to be a multi-circuit DC system.

[0006] Preferably, S100 further includes: Monitor real-time data from each converter station, extract the DC network topology from system or design documents, and establish a hybrid power flow model containing DC. If all DC power is concentrated on a single line, it is determined to be a single-circuit DC system; if the power is dispersed across multiple lines and has complementary characteristics, it is determined to be a multi-circuit DC system.

[0007] Preferably, S100 further includes: If an N-1 fault is triggered through simulation or historical data, and the remaining DC branch power increase is ≥ 80% of the fault branch capacity, the system is determined to be a multi-circuit DC system; otherwise, it is determined to be a single-circuit DC system.

[0008] Preferably, S100 further includes: A weight threshold is set. If the total score of multiple-loop features is ≥ 0.7, it is determined to be a multi-loop DC system; otherwise, it is determined to be a single-loop DC system.

[0009] Preferably, the effective system strength ratio SCESSR is calculated as follows: , in, is the short-circuit capacity of the AC busbar at the receiving end of the single-circuit DC system, is the dynamic reactive power compensation capability of the converter station, is the rated transmission power of the DC system, and k is the dynamic reactive power weighting coefficient.

[0010] Preferably, the dynamic reactive weighting coefficient k is set to 1.0 if the converter adopts fast voltage control; and to 0.5 if only slow reactive compensation is provided.

[0011] Preferably, the effective system strength ratio SCESSR>1.5: the system strength is sufficient, the voltage stability is high, and the fault recovery capability is strong; 1.0≤ SCESSR ≤ 1.5: The system strength is critical and dynamic control measures are required to maintain stability; SCESSR < 1.0: The system strength is insufficient, and there is a risk of voltage collapse or power transmission limitation.

[0012] Preferably, the effective synergistic intensity ratio MCESSR is calculated as follows: , in, is the AC bus short-circuit capacity of the i-th converter station connection point, is the dynamic reactive power compensation capability of the i-th converter station, is the reactive contribution efficiency coefficient of the i-th converter station, is the rated transmission power of i loops, is the redundant power coordination term, is the redundant power that can be transferred by the system, is a synergistic factor.

[0013] Preferably, the effective coordination strength ratio MCESSR>2.0: the system has high coordination strength, sufficient dynamic support between multiple circuits, and strong fault tolerance; 1.2 ≤ MCESSR ≤ 2.0: The synergy strength is moderate and needs to rely on control strategy optimization to maintain stability; MCESSR < 1.2: Insufficient coordination strength, with the risk of cascading failures or power blocking.

[0014] Preferably, the present invention proposes a method for evaluating the ability of a controllable commutated converter to improve power system stability. First, a DC transmission system model is established using the PSCAD / EMTDC simulation tool. Then, a short-circuit fault (such as a three-phase fault) is set in a single-circuit branch. The system's stability improvement is verified by observing the voltage and power recovery times. A simultaneous multi-circuit fault (such as a two-circuit DC blocking fault) is set in a multi-circuit branch. The system's stability improvement is verified by observing the power increase and voltage stability of the remaining circuits.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. It can provide a method for CLCC to evaluate its ability to improve power system stability.

[0016] 2. The two proposed indices: the "Significant System Strength Ratio (SCESSR)" of a single-circuit DC system and the "Meanwhile Coordinated Strength Ratio (MCESSR)" of a multi-circuit DC system, can both improve the assessment of power system stability and, to a certain extent, be helpful in power system planning. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the overall model of CLCC simulation in one embodiment of the present invention; Figure 2 It is a specific flow chart in one embodiment of the present invention; Figure 3 This is a flow chart of a method for evaluating the effect of a controllable commutated converter on improving the stability of a power system according to an embodiment of the present invention; Figure 4is a hybrid power flow model including DC in one embodiment of the present invention; Figure 5 1 is a schematic diagram of power boosting of the remaining DC branch in one embodiment of the present invention; Figure 6 This is a single-circuit DC power system model in one embodiment of the present invention; Figure 7 is a multi-circuit DC power system model in one embodiment of the present invention; Figure 8 It is a schematic diagram of the conduction process of a controllable phase-changing valve in one embodiment of the present invention. DETAILED DESCRIPTION

[0018] The following is a combination of the embodiments of the present invention Figures 1 to 8 , clearly and completely describes the technical solutions in the embodiments of the present invention; obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0019] In the description of the present invention, it should be noted that the terms "upper", "lower", "left", "right", "top / bottom" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limiting the present invention.

[0020] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "provided with," "mounted / connected," and "connected" should be understood in a broad sense. For example, "connected" can mean a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be internal communication between two components. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention in specific circumstances.

[0021] The present invention provides the following technical solutions: A method for evaluating the ability of a controllable commutated converter to improve power system stability, the method comprising: S100. Determine whether the DC system in the power grid is a single-circuit DC system or a multi-circuit DC system; S200. Build a single-circuit DC power system model, propose an effective system strength ratio, and evaluate the stability improvement effect in the single-circuit DC system; S300. Build a multi-circuit DC power system model, propose an effective coordination intensity ratio, and evaluate the stability improvement effect of the multi-circuit DC system; S400. Use simulation tools to verify the accuracy of the stability improvement effect.

[0022] In one embodiment, the CLCC converter model includes a plurality of controllable phase-commutating valves, such as Figure 1 As shown, the three-phase AC power supply is connected to the converter bridge through a Y-type connection. The converter bridge consists of six controllable phase-changing valves (V1, V2, V3, V4, V5, and V6). The controllable phase-changing valves are turned on and off according to a certain triggering sequence to realize the conversion of AC power to DC power.

[0023] Each controllable phase-commutation valve consists of a main branch and an auxiliary branch. The main branch, consisting of a high-voltage, high-current thyristor valve and a low-voltage, high-current IGBT valve, is responsible for the primary power transmission task. The auxiliary branch, consisting of a high-voltage, low-current auxiliary valve, provides auxiliary control and protection functions.

[0024] Under normal operating conditions, the thyristor valves in the main branch switch on and off according to a predetermined triggering sequence, achieving efficient power transmission. In the event of a system failure or when special control is required, the auxiliary branch can respond quickly and provide additional control and protection functions.

[0025] Furthermore, the conduction process is as follows Figure 8 As shown, in the initial state, V1 and V2 are turned on, and the current flows from phase A of the power supply into the load through V1, and then returns to point N of the power supply through V2.

[0026] When the voltage on phase B of the power supply becomes higher than that on phase A, the commutation process begins. V3 and V2 gradually turn on, and current begins to shift from phase A to phase B. During the commutation process, the circuit can be simplified into an equivalent circuit for analysis, namely the commutation equivalent circuit.

[0027] After the commutation is completed, the current flows from phase B of the power supply into the load through V3, and then returns to point N of the power supply through V2.

[0028] Figure 8 The timing diagram below shows the conduction of the six controllable phase-commutated valves (V1 to V6) at different time intervals.

[0029] In another embodiment, S100 includes: S101: Based on the topological feature analysis, the nodes of the converter stations are obtained. If there are only two converter stations in the power grid (one at the sending end and one at the receiving end) and the DC lines are point-to-point connected, it is preliminarily determined to be a single-circuit DC system (feature F1). If there are ≥3 converter stations or multiple groups of independent converter units are configured in the same converter station (such as two groups of 12-pulse converters), it is determined to be a multi-circuit DC system (feature F2).

[0030] In another embodiment, S100 further includes: S102: Monitor the real-time data of each converter station and extract the DC network topology from the SCADA / EMS system or design documents, including the location of the converter station, the DC line connection relationship and equipment parameters, and obtain the power, voltage, and current monitoring values ​​of the DC line, as well as the voltage and phase angle information of the AC node.

[0031] S102-1: Establish a hybrid power flow model with DC, such as Figure 4 As shown in Figure 1, power is transmitted between the sending-end converter station and the receiving-end converter station via a ±500kV DC line. The sending-end converter station is preferably a VSC (voltage source converter), LCC (linear commutation converter) or CLCC (controlled commutation converter), and its transmission power P send -1000MW, dynamic reactive power Q dyn The receiving converter station is preferably VSC, LCC or CLCC, and its receiving power P recv 980MW, dynamic reactive power Q dyn ±200Mvar.

[0032] The sending-end converter station and the receiving-end converter station are connected to their respective AC busbars through the 230kV AC side. The short-circuit capacity S of the sending-end AC busbar is sc The short-circuit capacity S of the receiving AC bus is 5000MVA and the voltage per unit value V is 1.02pu. sc The same is 5000MVA, and the voltage per unit value V is also 1.02pu.

[0033] The sending-end AC busbar is connected to an AC system consisting of generators, loads, and the network, and ultimately to the AC grid. The receiving-end AC busbar is connected to an AC system consisting of loads and a STATCOM (Static Synchronous Compensator), and ultimately to the AC grid.

[0034] Single-circuit system model: The DC branch is equivalent to a PQ node pair, satisfying the following: , where P send Indicates the output power of the sending-end converter station; P recv Indicates the power absorbed by the receiving converter station; P loss Indicates the current loss.

[0035] Multi-loop system model: embed multi-loop coordination equations, such as , where P dc,i Represents the power of each converter station node; P total Indicates total power.

[0036] S102-2: Based on power characteristic analysis, if all DC power is concentrated on a single line (fluctuation range ≤ rated value ±5%), it is determined to be a single-circuit DC system (characteristic F3). If the power is dispersed across multiple lines and there are complementary characteristics (such as when one line is fully loaded, another line automatically increases its capacity), it is determined to be a multi-circuit DC system (characteristic 4).

[0037] Here is an example: Single-circuit DC system: Gezhouba-Nanqiao DC project The total transmission capacity of 3,000 MW relies entirely on a single line. Historical operating data shows that power fluctuations have always been controlled within a range of ±150 MW (±5%).

[0038] Multi-circuit DC system: Jinsha River hydropower transmission project (Xiangjiaba / Xiluodu to Zhejiang DC group) The total power of 6400MW is distributed across two independent DC lines (each rated at 3200MW).

[0039] The two lines show a significant negative correlation: when the Xiluodu DC is fully loaded, the Xiangjiaba DC automatically reduces its load, and vice versa (as shown in the table below).

[0040]

[0041] In another embodiment, S100 further includes: S103: Based on fault response analysis, since single-circuit DC systems can only detect a single power / voltage control command (such as constant power mode), while multi-circuit systems can detect multi-objective optimization commands (such as power balancing and voltage droop control), an N-1 fault is triggered using simulation or historical data (assuming a branch fails and stops operating). If the power increase of the remaining DC branches is ≥ 80% of the capacity of the faulty branch, the system is determined to be a multi-circuit DC system (feature F5).

[0042] Specifically, when N-1 faults occur in a DC branch, the power increase of the remaining branches must meet the following requirements: ∑Δ P boost,i ≥0.8· P fault Among them, Δ P boost,i For the i The power increase of the remaining branches (MW); P fault is the rated capacity of the faulty branch (MW). If the above conditions are met, the system is a multi-circuit DC system; otherwise, it is a single-circuit system.

[0043] Example: System topology such as Figure 5As shown, among them, the converter stations are: A (sending end), B and C (receiving end); DC branch: A→B (original power P dc,A→B =1000 MW), A→C (original power P dc,A→C =800 MW), B→C (original power P dc,B→C =600 MW); Redundant power: Each branch can be overloaded by a maximum of 20% (i.e. γ =1.2).

[0044] Fault scenario: Assume that branch A→B fails ( P fault =1000MW), the power increase capacity of the remaining branches A→C and B→C needs to be calculated.

[0045] Calculation process: Step 1: Maximum allowable power of remaining branches Branch A→C: P max , A→C = γ ⋅ P dc,A→C =1.2×800=960MW Branch B→C: P max,B→C = γ ⋅ P dc , B→C =1.2×600=720MW Step 2: Actual Power Boost The original power of branch A→C is 800 MW, which can be increased by: Δ P boost , A→C =960−800=160MW The original power of branch B→C is 600 MW, which can be increased by: Δ P boost,B→C =720−600=120MW Step 3: Total power increase ∑Δ P boost =160+120=280MW Step 4: Verify the judgment conditions 0.8⋅ P fault =0.8×1000=800MW Since 280 MW < 800 MW, the judgment conditions are not met, so it is determined to be a single-circuit DC system.

[0046] In another embodiment, S100 further includes: S104: Set weight thresholds (for example, topology feature A has a weight of 40%, power feature B has a weight of 30%, and fault response C has a weight of 30%). If the total score of the multi-circuit features, M, equals 0.4A + 0.3B + 0.3C, and is greater than or equal to 0.7, the system is determined to be a multi-circuit DC system. Otherwise, the system is determined to be a single-circuit DC system.

[0047] In another embodiment, S200 includes: S201: Use the simulation software PSCAD to build a detailed single-circuit DC power system model based on the CLCC topology, such as Figure 6 As shown, adjust the parameters to make the operating state of the controllable commutation converter consistent with the actual situation. Example: At the sending end, the AC grid provides 230kV, 50H z The AC power is input to the sending converter station. The CLCC converter in the sending converter station converts the AC power into ±500kV DC power, which is then transmitted to the remote receiving converter station via the HVDC transmission line at a power of 1000MW.

[0048] At the receiving end, the CLCC converters in the receiving converter station convert the ±500kV DC power back into 230kV, 50Hz AC power. The converted AC power is then connected to the receiving end's AC grid via transmission lines for consumption.

[0049] S202: To calculate the ability to improve the stability of a single-circuit DC system, the effective system strength ratio (SCESSR) is proposed to quantify the matching relationship between the AC system strength and the DC system operating capability. It is defined as:

[0050] : Short-circuit capacity of the AC busbar at the receiving end of a single-circuit DC system (MVA); : Dynamic reactive power compensation capability (Mvar) of the converter station, including the reactive power regulation range of STATCOM, SVG or the converter itself; : rated transmission power of DC system (MW); k: Dynamic reactive power weighting coefficient (usually 0.5~1.0, reflecting the contribution efficiency of reactive support to system strength).

[0051] The physical meaning of the effective system strength ratio (SCESSR) is: SCESSR>1.5: The system has sufficient strength, high voltage stability, and strong fault recovery capability; 1.0≤ SCESSR ≤ 1.5: The system strength is critical and dynamic control measures are required to maintain stability; SCESSR < 1.0: The system strength is insufficient, and there is a risk of voltage collapse or power transmission limitation.

[0052] Furthermore, the system strength is sufficient → voltage stability is high: AC bus voltage U By injection current I Decide:

[0053] High system strength S sc Large, so the voltage fluctuation is smaller under the same current change (Δ U ∝1 / S sc ).

[0054] Sufficient system strength → Strong fault recovery capability: 1. Fault current support: 1) Strong system: short-circuit current I sc = U n / Z sys , I sc The larger the value, the more sensitive the protection device will be and the action time will be shortened by 20%~40%. n is the rated voltage.

[0055] 2) Weak system: I sc If the fault current is too low, the protection may fail to operate and the fault clearing may be delayed (for example, a wind farm grid disconnection accident was delayed by 200ms due to insufficient fault current).

[0056] 2. Transient kinetic energy reserve: 1) System kinetic energy E k =1 / 2Jω 2 (J: equivalent moment of inertia) 2) High SCR systems are usually accompanied by large-capacity synchronous machines → J is large → the frequency change rate df / dt during fault is low (as shown in the table below).

[0057]

[0058] 3. Self-healing mechanism activation: 1) Strong system: Rapid voltage recovery → Triggering SVG / SVC dynamic reactive power compensation (response time <50ms).

[0059] 2) Weak system: Voltage remains low → Reactive equipment cannot start (needs to rely on slow-speed phase regulators).

[0060] The proposed SCESSR quantifies the relationship between AC system strength and DC system operating capacity, and verifies stability. It also provides a quantitative metric for evaluating the voltage stability of a single-circuit DC system, helping to determine whether dynamic control measures are needed to maintain stability.

[0061] Further: S202-1: Collect data to obtain the short-circuit capacity of the AC busbar at the receiving end (Through short-circuit calculation or actual measurement); extract the dynamic reactive power compensation capacity of the converter station (e.g. VSC reactive power regulation range ±200Mvar); Determine DC rated power (e.g. 1000MW).

[0062] S202-2: Calibrate the k value according to the control strategy effect: If the converter adopts fast voltage control (response time <50ms), take k=1.0; if it only has slow reactive power compensation (response time >200ms), take k=0.5.

[0063] S202-3: Calculate SCESSR; S202-4: Perform stability verification by simulating an AC fault (such as a three-phase short circuit) in PSCAD / EMTDC and observing the following indicators: voltage drop during the fault (target: ≥70% per unit) and power recovery time (target: <200ms).

[0064] Exemplary: 1. SCESSR>1.5: System strength is sufficient Parameter setting: Receiving end AC bus short circuit capacity S sc =5000 MVA; Dynamic reactive power compensation capability of converter station Q dyn =300Mvar; dynamic reactive power weighting coefficient k =1.0 (fast voltage control, response time <50ms); DC system rated power P dc =1000MW.

[0065] =(5000+1.0×300) / 1000=5.3 SCESSR = 5.3>1.5, the system strength is sufficient, the voltage stability is high, and the fault recovery capability is strong.

[0066] 2. 1.0 ≤ SCESSR ≤ 1.5: System strength critical Parameter setting: receiving end AC bus short-circuit capacity S sc=2000MVA; dynamic reactive power compensation capability of converter station Q dyn =200Mvar; dynamic reactive power weighting coefficient k =0.8 (medium-speed reactive power compensation, response time ≈100ms); DC system rated power P dc =2000MW.

[0067] =1.08 SCESSR = 1.08 ∈ [1.0, 1.5], the system strength is critical and needs to rely on dynamic control measures to maintain stability.

[0068] 3. SCESSR < 1.0: Insufficient system strength Parameter setting: receiving end AC bus short-circuit capacity S sc =800MVA; dynamic reactive power compensation capability of converter station Q dyn =100Mvar; dynamic reactive power weighting coefficient k =0.5 (slow reactive power compensation, response time>200ms); DC system rated power P dc =1000MW.

[0069] =0.85 SCESSR = 0.85<1.0, the system strength is insufficient, and there is a risk of voltage collapse or power transmission limitation.

[0070] In another embodiment, S300 includes: S301: Use the simulation software PSCAD to build a detailed multi-circuit DC power system model based on the CLCC topology, such as Figure 7 As shown, parameters are adjusted to ensure that the operating state of the controllable commutated converter is consistent with reality. For example, converter station A serves as the sending end, with a rated power of 1000 MW and a dynamic reactive capacity of ±200 Mvar. Converter station A is connected to converter station B via DC line 1 (±500 kV).

[0071] Converter Station B, the receiving terminal, has a rated power of 800 MW and a dynamic reactive capacity of ±150 Mvar. Converter Station B is connected to Converter Station D via DC Line 3 (±500 kV).

[0072] Converter Station C, the receiving terminal, has a rated power of 1200 MVA and a dynamic reactive capacity of ±250 Mvar. It is connected to Converter Station A via DC Line 2 (±500 kV), and ultimately to the AC grid (Region 1), which operates at 230 kV and has a short-circuit capacity of 6000 MVA.

[0073] Converter Station D, the receiving terminal, has a rated power of 600 MVA and a dynamic reactive capacity of ±100 Mvar. Converter Station D is connected to Converter Station B via DC Line 3 and ultimately to the AC grid (Region 2), which has a voltage of 230 kV and a short-circuit capacity of 4000 MVA. S302: To calculate the stability improvement capability of a multi-circuit DC system, the effective coordination strength ratio (MCESSR) is proposed. This quantifies the comprehensive matching relationship between AC system strength, dynamic reactive coordination capability, and redundant support in a multi-circuit system. It is defined as:

[0074] : AC bus short-circuit capacity of the i-th converter station connection point (MVA); : dynamic reactive power compensation capability of the i-th converter station (Mvar); : reactive contribution efficiency coefficient of the i-th converter station (0.5~1.0, determined by the control response speed); : Rated transmission power of i circuit (MW); : Redundant power coordination term, is the system's transferable redundant power (MW), is the synergy factor (0.2~0.5, reflecting the power transfer efficiency during fault conditions).

[0075] The physical meaning of the effective synergy strength ratio (MCESSR) is: MCESSR>2.0: The system has high coordination strength, sufficient dynamic support between multiple circuits, and strong fault tolerance; 1.2 ≤ MCESSR ≤ 2.0: The synergy strength is moderate and needs to rely on control strategy optimization to maintain stability; MCESSR < 1.2: Insufficient coordination strength, with the risk of cascading failures or power blocking.

[0076] Furthermore, P red Represents the power capacity that can be transferred by the remaining circuit in the event of a fault (overload capacity + reserve capacity).

[0077] 1) When a circuit fails, the remaining circuits take on the load of the faulty circuit through power redistribution (e.g., increasing the overload by 20%).

[0078] 2) High MCESSR → P red Large → Transferable power ≥ 80% fault power → Avoid load loss.

[0079] Dynamic reactive coordination is the dynamic reactive compensation capability k of the converter station. i Quantify its response speed (0.5~1.0).

[0080] 1) When a fault causes a voltage drop, multiple converter stations inject reactive power simultaneously (such as STATCOM operation).

[0081] 2) High MCESSR → Large → Faster voltage recovery (e.g. <300 ms) → Suppresses voltage collapse.

[0082] System strength support is the short-circuit capacity at the converter station access point, reflecting the inherent strength of the AC power grid.

[0083] 1) High short-circuit capacity provides strong voltage support and reduces the risk of fault propagation.

[0084] 2) High MCESSR → Large → Small voltage fluctuation after fault (e.g. drop ≤ 10%) → Avoid cascading tripping.

[0085] This paper proposes a MCESSR (Meanwhile Effective Coordination Strength Ratio) to quantify the comprehensive relationship between AC system strength, dynamic reactive power coordination, and redundant support in multi-circuit systems, and to verify their stability. This not only improves the accuracy of multi-circuit system stability assessment, but also enhances the adequacy of dynamic support between multiple circuits and improves fault tolerance.

[0086] Further, S302-1: Collect data and obtain the short-circuit capacity of all converter stations , reactive power compensation capability , rated transmission power ; Calculate the maximum transferable power through N-1 safety check (For example, the maximum capacity increase of the remaining circuits when a certain circuit fails). The calculation formula is as follows:

[0087] in, γ : Converter station overload factor (usually 1.2, i.e. allowing 20% ​​overload); S sc,i : No. i Short-circuit capacity (MVA) of each converter station connection point; Q dyn,i : No. i Dynamic reactive power compensation capability of each converter station (Mvar); k i : Dynamic reactive efficiency coefficient (0.5~1.0, determined by the control response speed); P dc,i : No. i Rated transmission power of each converter station (MW).

[0088] Exemplary: Assume a DC transmission system with three converter stations, with the following parameters: Converter station 1: Ssc,1=3000MVA, Qdyn,1=200Mvar, k1=1.0 (fast response), Pdc,1=1000MW; Converter station 2: Ssc,2=2500MVA, Qdyn,2=150Mvar, k2=0.8 (medium-speed response), Pdc,2=800MW; Converter station 3: Ssc,3=2000MVA Qdyn,3=100Mvar, k3=0.5 (slow response), Pdc,3=600MW.

[0089] Assume that converter station 3 fails (j=3), and calculate the Pred of the remaining converter stations 1 and 2.

[0090] Converter Station 1: = 200MW; Converter Station 2: = 160MW.

[0091] S302-2: Calibrate the k value according to the effect of the control strategy: If the converter adopts fast voltage control (response time <50ms), take k=1.0; if it only has slow reactive power compensation (response time >200ms), take k=0.5.

[0092] Defining the synergistic factor : High coordination (e.g. DC grid with fast power routing): =0.5; low coordination (such as multiple independently controlled lines): =0.2.

[0093] S302-3: Calculate MCESSR; S302-4: Perform stability verification and simulate multiple circuit simultaneous faults (such as two-circuit DC blocking) in PSCAD / EMTDC. Figure 7As shown, observe the power increase and voltage stability of the remaining circuits and the following indicators: After the fault, MCESSR is still 1.2, voltage recovery time <300ms.

[0094] Exemplary: 1. MCESSR>2.0: High system synergy strength Parameter settings: 1) Converter Station 1: S sc,1 =4000MVA, Q dyn,1 =300Mvar, k 1=1.0 (fast response), P dc,1= 1000MW 2) Converter Station 2: S sc,2 =3500MVA, Q dyn,2 =250Mvar, k 2=1.0, P dc,2 =1000MW 3) Converter Station 3: S sc,3 =3000MVA, Q dyn,3 =200Mvar, k 3=1.0, P dc,3 =1000MW 4) Redundant power: P red =1000MW, α =0.5 (high synergy) ≈3.92 MCESSR = 3.92>2.0, the system has high coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance.

[0095] 2. 1.2 ≤ MCESSR ≤ 2.0: Moderate synergistic strength Parameter settings: 1) Converter Station 1: S sc,1 =2000MVA, Q dyn,1 =200Mvar, k 1=0.8 (medium speed response), P dc,1 =800MW 2) Converter Station 2: Ssc,2 =1800MVA, Q dyn,2 =150Mvar, k 2=0.8, P dc,2 =1200MW 3) Redundant power: P red =500MW, α =0.3 (low synergy) ≈2.12 MCESSR = 2.12 ∈ [1.2, 2.0], the synergy strength is medium, and it needs to rely on control strategy optimization to maintain stability.

[0096] 3. MCESSR < 1.2: Insufficient synergistic strength Parameter settings: 1) Converter Station 1: S sc,1 =1000MVA, Q dyn,1 =100Mvar, k 1=0.5 (slow response), P dc,1 =1500MW 2) Converter Station 2: S sc,2 =800MVA, Q dyn,2 =50Mvar, k 2=0.5, P dc,2 =500MW 3) Redundant power: P red =300MW, α =0.2 (independent control) ≈0.97 MCESSR = 0.97<1.2, the synergy strength is insufficient, and there is a risk of cascading failure or power blocking.

[0097] In another embodiment, the present invention provides a method for evaluating the effect of a controllable commutated converter on improving the stability of a power system, such as Figure 3 As shown: First, determine whether the system contains a single-circuit DC branch or multiple-circuit DC branches. Calculate the total score of the multiple-circuit characteristics based on the set weight thresholds (topology characteristics: 40%, power characteristics: 30%, and fault response: 30%). If the total score is ≥ 0.7, proceed to the "multiple-circuit branch" branch; otherwise, proceed to the "single-circuit branch" branch.

[0098] Single-circuit branch: Data collection: collect data such as short-circuit capacity, reactive power compensation capacity, and DC rated power; Calibrate k value: calibrate a coefficient k based on the collected data; Calculate the effective system strength ratio SCESRR; If SCESRR>1.0, the system is considered to have sufficient strength and strong stability improvement capabilities; otherwise, the system is considered to have insufficient strength and there is a risk of voltage collapse.

[0099] Multi-circuit branch: Data collection: also collects data such as short-circuit capacity, reactive power compensation capability, and rated transmission power; Calibrate k value: calibrate a coefficient k based on the collected data; Calculate the effective synergy intensity ratio MCESRR; If MCESRR>1.2, it is considered that the system coordination strength is high, the dynamic support between multiple loops is sufficient, and the fault tolerance capability is strong; otherwise, it is considered that the coordination strength is insufficient and there is a risk of cascading failures or power blocking.

[0100] Figure 3 This method provides a systematic approach to assessing DC system types and their stability. By calculating a weighted total characteristic score, it can accurately distinguish between single-circuit and multi-circuit systems. Combined with the collection and analysis of specific data, it can quantitatively assess the system's stability and coordination capabilities, providing a scientific basis for grid planning and operation.

[0101] The above are only preferred embodiments of the present disclosure and are not intended to limit the implementation methods and protection scope of the present disclosure. Those skilled in the art should be aware that any solutions obtained by equivalent substitutions and obvious changes made using the contents of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A method for evaluating the ability of a controllable commutated converter to improve the stability of a power system, characterized in that: The method comprises: S100. Determine whether the DC system in the power grid is a single-circuit DC system or a multi-circuit DC system; S200. Build a single-circuit DC power system model, propose an effective system strength ratio, and evaluate the stability improvement effect in the single-circuit DC system; S300. Build a multi-circuit DC power system model, propose an effective coordination intensity ratio, and evaluate the stability improvement effect of the multi-circuit DC system; S400. Use simulation tools to verify the accuracy of the stability improvement effect.

2. The method according to claim 1, characterized in that S100 includes: Based on the topological feature analysis, the nodes of the converter stations are obtained. If there are only two converter stations in the power grid and the DC lines are point-to-point connected, it is determined to be a single-circuit DC system. If there are ≥3 converter stations or multiple groups of independent converter units are configured in the same converter station, it is determined to be a multi-circuit DC system.

3. The method according to claim 1, characterized in that The S100 also includes: Monitor real-time data from each converter station, extract the DC network topology from system or design documents, and establish a hybrid power flow model containing DC. If all DC power is concentrated on a single line, it is determined to be a single-circuit DC system; if the power is dispersed across multiple lines and has complementary characteristics, it is determined to be a multi-circuit DC system.

4. The method according to claim 1, wherein The S100 also includes: If an N-1 fault is triggered through simulation or historical data, and the remaining DC branch power increase is ≥ 80% of the fault branch capacity, the system is determined to be a multi-circuit DC system; otherwise, it is determined to be a single-circuit DC system.

5. The method according to claim 1, characterized in that The S100 also includes: A weight threshold is set. If the total score of multiple-loop features is ≥ 0.7, it is determined to be a multi-loop DC system; otherwise, it is determined to be a single-loop DC system.

6. The method according to claim 1, characterized in that The calculation formula of the effective system strength ratio SCESSR is: ， in, is the short-circuit capacity of the AC busbar at the receiving end of the single-circuit DC system, is the dynamic reactive power compensation capability of the converter station, is the rated transmission power of the DC system, and k is the dynamic reactive power weighting coefficient.

7. The method according to claim 6, characterized in that The dynamic reactive power weighting coefficient k is set to 1.0 if the converter adopts fast voltage control; and to 0.5 if only slow reactive power compensation is provided.

8. The method according to claim 6, characterized in that The effective system strength ratio SCESSR > 1.5 indicates sufficient system strength, high voltage stability, and strong fault recovery capability. 1.0≤ SCESSR ≤ 1.5: The system strength is critical and dynamic control measures are required to maintain stability; SCESSR < 1.0: The system strength is insufficient and there is a risk of voltage collapse or power transfer being limited.

9. The method according to claim 1, characterized in that The effective synergistic intensity ratio MCESSR is calculated as follows: ， in, is the AC bus short-circuit capacity of the i-th converter station connection point, is the dynamic reactive power compensation capability of the i-th converter station, is the reactive contribution efficiency coefficient of the i-th converter station, is the rated transmission power of i loops, is the redundant power coordination term, is the redundant power that can be transferred by the system, is a synergistic factor.

10. The method according to claim 1, characterized in that The effective coordination strength ratio MCESSR> 2.0: the system has high coordination strength, sufficient dynamic support between multiple circuits, and strong fault tolerance; 1.2 ≤ MCESSR ≤ 2.0: The synergy strength is moderate and needs to rely on control strategy optimization to maintain stability; MCESSR < 1.2: Insufficient coordination strength, with the risk of cascading failures or power blocking.

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