A station-level reactive power control method of a flexible direct current power transmission system

Abstract

This invention discloses a station-level reactive power control method for a flexible DC transmission system, comprising: detecting and obtaining measured values ​​of voltage and current signals at the converter station's point of common coupling (PCC); comparing the reference value of the AC current reactive component at PCC with the q-axis component of the measured current signal, the difference being used as the reactive current deviation at PCC; obtaining the voltage compensation amount for the AC near-zone of the converter station after applying a correction factor of K times to the reactive current deviation; comparing the AC voltage reference value with the voltage compensation amount and the measured AC voltage value to obtain the reactive power outer loop deviation; obtaining the AC current reactive component reference value after PI regulation of the reactive power outer loop deviation; and using the AC current reactive component reference value as the input for inner loop control. This method can fully utilize the reactive power regulation capability of the flexible DC transmission system, reduce the frequency of fluctuations during reactive power compensation, and prevent drastic fluctuations caused by reactive power compensation.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission technology, specifically relating to a station-level reactive power control method for a flexible DC power transmission system. Background Technology

[0002] With the development of flexible DC transmission systems towards higher voltage levels, larger transmission capacities, multi-terminal deployments, and network integration, these flexible and reliable systems have broad application prospects. Existing station-level control strategies for flexible DC systems are mostly active-reactive power decoupling control, with a single reactive power control component. Traditional reactive power control methods mainly include constant reactive power control and constant AC voltage control. Specifically:

[0003] When a converter station is connected to an AC system, under normal operating conditions, accurately transmitting active power is generally the highest priority. When voltage fluctuations occur at the AC nodes connected to the converter station, these fluctuations need to be suppressed quickly to prevent disruption to active power transmission. In this situation, using traditional constant reactive power control with a reactive power reference value of 0 is ineffective in suppressing voltage fluctuations and also fails to fully utilize the flexible and independent control characteristics of active and reactive power in flexible DC transmission, thus neglecting the reactive power regulation capabilities of the flexible DC transmission system. While traditional constant AC voltage control can suppress voltage fluctuations, this method suppresses them too frequently, potentially leading to frequent reactive power adjustments. This hinders rapid system voltage recovery and makes it difficult for reactive power compensation equipment near the converter station to coordinate and for the rational allocation of reactive power. Summary of the Invention

[0004] To address the problems in the prior art, this invention provides a station-level reactive power control method for flexible DC transmission systems.

[0005] The technical solution of the present invention is as follows:

[0006] A station-level reactive power control method for a flexible DC transmission system includes:

[0007] The measured value U of the voltage signal at the converter station's common coupling point was obtained. s and the measured value of the current signal I s ;

[0008] Reference value I of reactive component of AC current at converter station common coupling point sqref Measured value of current signal I s q-axis component I sq The difference is compared and taken as the reactive current deviation ΔI at the converter station's point of common coupling. sq ΔI sq The voltage compensation amount ΔU in the AC near-zone of the converter station is obtained after adjusting the gain by a factor of K.ref ;

[0009] AC voltage reference value U ref With voltage compensation amount ΔU ref and measured AC voltage U s After comparison, the reactive power outer loop deviation ΔU is obtained. After ΔU is adjusted by PI, the reference value I of the reactive power component of the AC current is obtained. sqref The reference value I of the reactive component of the alternating current. sqref As input for inner loop control.

[0010] Furthermore, the correction coefficient K = 100l, where l is the reactive power compensation participation coefficient of the converter station, and the value range of l is (0,1).

[0011] Furthermore, the voltage compensation amount ΔU in the AC near-field of the converter station ref The calculation formula is as follows:

[0012] ΔU ref =K*ΔI sq .

[0013] Furthermore, the formula for calculating the reactive power outer loop deviation ΔU is as follows:

[0014] ΔU=U ref -ΔU ref -U s .

[0015] Furthermore, the AC voltage reference value U ref Take the voltage rating U at the converter station's common coupling point set .

[0016] Furthermore, the station-level active power control of the flexible DC transmission system adopts constant active power control or constant DC voltage control.

[0017] Furthermore, the constant active power control includes the following specific methods:

[0018] The measured active power P at the converter station's common coupling point was obtained. s The measured value P of the detected active power s With active power reference value P ref The difference is compared and then adjusted by a PI controller to obtain the reference value I for the active component of the AC current. sdref The reference value I of the reactive component of the alternating current. sdref As input for inner-loop control;

[0019] The constant DC voltage control includes the following specific methods:

[0020] The measured value U of the DC line voltage of the converter station was obtained. dcThe measured value U of the detected DC line voltage dc DC line voltage reference value U dcref The difference is compared and then adjusted by a PI controller to obtain the reference value I for the active component of the AC current. sdref The reference value I of the active component of alternating current. sdref As input for inner loop control.

[0021] Furthermore, in the inner loop control, the d-axis component I of the measured current signal is first... sd and the q-axis component I of the measured current signal sq Compared with the reference value I of the active component of alternating current respectively sdref and the reference value I of the reactive component of the alternating current. sqref The differences are compared and then PI-adjusted. The adjusted corrections are then compared with the coupling terms between the d-axis and q-axis currents, as well as the d-axis component U of the measured voltage signal. sd q-axis component U of the measured voltage signal sq The calculations were performed to obtain the d-axis component U of the converter station outlet voltage. cd and the q-axis component of the converter station outlet voltage U cq Then, the d-axis component U of the converter station outlet voltage... cd and the q-axis component of the converter station outlet voltage U cq Input SPWM.

[0022] Furthermore, the d-axis component I of the measured current signal sd and the q-axis component I of the measured current signal sq It is by measuring the current signal I s Obtained by performing a dq transform;

[0023] Furthermore, the d-axis component U of the measured voltage signal value sd q-axis component U of the measured voltage signal sq It is achieved by measuring the voltage signal U at the converter station's common coupling point. s It is obtained by performing a dq transformation.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] This invention proposes a station-level reactive power control method for flexible DC transmission systems. Compared to traditional constant reactive power control, this method uses the stable voltage at the point of common coupling as the control objective, with the AC voltage reference value U... ref Take the voltage rating U at the converter station's common coupling point set This allows for full utilization of the reactive power regulation capability of the flexible DC transmission system, and facilitates the application of the flexible DC transmission system's characteristics of flexible and independent control of active and reactive power.

[0026] The station-level reactive power control method of this invention, compared with traditional AC voltage control, adds a reactive current compensation stage to compensate for the reactive current deviation ΔI. sq The voltage compensation amount ΔU in the AC near-zone of the converter station is obtained after adjusting the gain by a factor of K. ref This design is used for reactive current compensation. It can reduce the frequency of fluctuations during reactive current compensation and prevent drastic fluctuations caused by reactive current compensation. At the same time, the invention designs the value of the correction coefficient K and associates it with the reactive current compensation participation coefficient of the converter station. This is more conducive to cooperating with reactive current compensation equipment in the vicinity of the converter station and to the rational distribution of reactive power when reactive current compensation equipment is operating in parallel. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the station-level control structure of a DC transmission system.

[0028] Figure 2 This is a block diagram of active and reactive power decoupling control for a true bipolar flexible DC transmission system at the station level.

[0029] Figure 3 This is a schematic diagram of the reactive power control structure in station-level control.

[0030] Figure 4 This is a simulation curve of the DC voltage waveform of the converter station.

[0031] Figure 5 The voltage signal waveform simulation curve at the converter station's common coupling point is shown in the figure.

[0032] Among them, PCC - Common Connection Point of Converter Station; P s - Measured active power at the converter station's common connection point; P ref - Active power reference value at the converter station's common coupling point; U sd - The d-axis component of the measured voltage signal; U cd - D-axis component of converter station outlet voltage; M - Amplitude of converter station outlet voltage; SPWM - Sine pulse width modulation; I sd - The d-axis component of the measured current signal; I sq - The q-axis component of the measured current signal; I sqref - Reference value for reactive component of alternating current; U sq - The q-axis component of the measured voltage signal; U cq - q-axis component of converter station outlet voltage; δ - phase angle of converter station outlet voltage; U s -Measured voltage signal value at the converter station's common coupling point; ΔI sq - Reactive current deviation at the converter station's point of common coupling; K - Correction factor; ΔU ref - Voltage compensation amount in the AC near-zone of the converter station; U ref- AC voltage reference value; ΔU - reactive power outer loop deviation; U dc - DC voltage. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments and corresponding drawings.

[0034] Example 1:

[0035] This embodiment presents a station-level reactive power control method for a flexible DC transmission system, applicable to scenarios involving voltage fluctuations under normal operating conditions of the flexible DC transmission system. Figures 1-3 As shown, the reactive power control method at this station level mainly includes two stages:

[0036] I. Monitoring Stage: Used to detect and generate voltage and current signals at the converter station's common connection point;

[0037] II. Reactive Power Outer Loop Deviation Generation Stage: Based on the voltage and current signals from the monitoring stage, the corresponding reactive power outer loop deviation is generated as feedback input. This station-level reactive power control method specifically includes the following steps:

[0038] Measured value U of voltage signal at the common coupling point of the converter station s and the measured value of the current signal I s ;

[0039] Reference value I of reactive component of AC current at converter station common coupling point sqref Measured value of current signal I s q-axis component I sq The difference is compared and taken as the reactive current deviation ΔI at the converter station's point of common coupling. sq ΔI sq =I sqref -I sq ;

[0040] Reactive current deviation ΔI at the converter station common coupling point sq The voltage compensation amount ΔU in the AC near-zone of the converter station is obtained after adjusting the gain by a factor of K. ref Voltage compensation ΔU in the AC near-zone of the converter station ref The calculation formula is as follows:

[0041] ΔU ref =K*ΔI sq .

[0042] AC voltage reference value U ref With voltage compensation amount ΔU ref The difference after comparison is used as a reference value for reactive power parameters, and then compared with the measured value of AC voltage U. s By comparison, the reactive power outer loop deviation ΔU and the measured AC voltage U are obtained.s The actual value of the reactive power parameter. The formula for calculating the reactive power outer loop deviation ΔU is as follows:

[0043] ΔU=U ref -ΔU ref -U s .

[0044] Among them, the AC voltage reference value U ref Take the voltage rating U at the converter station's common coupling point set U ref =U set .

[0045] The aforementioned correction coefficient K = 100l, where l is the reactive power compensation participation coefficient of the converter station, and the value of l ranges from (0,1). The value of l depends on the proportion of the reactive power compensation of the converter station to the total reactive power compensation of the AC system connected to the converter station. The higher the proportion of the reactive power compensation of the converter station, the larger the value of l. The total reactive power compensation of the AC system connected to the converter station is the sum of the reactive power compensation of the converter station and the compensation amount of the reactive power compensation equipment in the AC system connected to the converter station.

[0046] The reactive power outer loop deviation ΔU is adjusted by PI13 to obtain a new AC current reactive power component reference value I. sqref The new AC current reactive component reference value I sqref As input for inner loop control.

[0047] Example 2:

[0048] This embodiment is a further design based on Embodiment 1. In this example, the station-level active power control of the flexible DC transmission system adopts constant active power control or constant DC voltage control. The specific choice can be made according to the strength of the connected AC system and the scheduling arrangement. Generally, a converter station with a strong connected AC system is selected to adopt constant DC voltage control, while the other end adopts constant active power control.

[0049] Active power control includes the following two specific methods:

[0050] Method 1: Measure the active power P at the converter station's common coupling point. s and P s As the actual value of the active power parameter, obtain the active power reference value P. ref and P ref As a reference value for active power; measured value of active power P s With active power reference value P ref The difference is compared and adjusted by PI11 to obtain the reference value I of the active component of the AC current. sdref The reference value I of the reactive component of the alternating current. sdref As input for inner loop control.

[0051] Method 2: Obtain the measured value U of the DC line voltage of the converter station. dc The measured value U of the detected DC line voltage dc DC line voltage reference value U dcref The difference is compared and then adjusted by a PI controller to obtain the reference value I for the active component of the AC current. sdref The reference value I of the active component of alternating current. sdref As input for inner loop control.

[0052] In the inner loop control of this example, the d-axis component I of the measured current signal is first... sd and the q-axis component I of the measured current signal sq Compared with the reference value I of the active component of alternating current respectively sdref and the reference value I of the reactive component of the alternating current. sqref The differences are compared and adjusted using PI12 and PI14 respectively. The adjusted corrections are then compared with the coupling terms between the d-axis and q-axis currents and the d-axis component U of the measured voltage signal. sd q-axis component U of the measured voltage signal sq The calculations were performed to obtain the d-axis component U of the converter station outlet voltage. cd and the q-axis component of the converter station outlet voltage U cq Then, the d-axis component U of the converter station outlet voltage... cd and the q-axis component of the converter station outlet voltage U cq After coordinate transformation, the voltage amplitude and phase angle are input to SPWM for modulation, generating the corresponding trigger pulse signal.

[0053] The d-axis component I of the above measured current signal sd and the q-axis component I of the measured current signal sq It is by measuring the current signal I s The d-axis component U of the measured voltage signal is obtained by performing a dq transformation. sd q-axis component U of the measured voltage signal sq It is by measuring the voltage signal U s It is obtained by performing a dq transformation.

[0054] Application Examples:

[0055] This embodiment simulates the station-level reactive power control method of the present invention on a true bipolar flexible DC transmission system. The converter stations of this true bipolar flexible DC transmission system operate independently in positive and negative directions. The positive pole adopts the station-level reactive power control method of the present invention, while the negative pole adopts traditional constant reactive power control. This embodiment establishes a model of the true bipolar DC transmission system and conducts simulation tests. The specific process is as follows:

[0056] Because a true bipolar converter station can achieve stable control of the AC voltage at the point of common coupling (PCC) using only one independent converter pole (voltage-controlled pole), both flexible DC transmission systems at both ends employ an active and reactive power decoupling control strategy. Based on the converter mathematical model in a synchronous rotating coordinate system (dq coordinates), a direct current control strategy and a dual closed-loop controller structure are used. In one converter station, active power control at both poles uses constant DC voltage control, and reactive power control uses constant reactive power control, with a reference value set to 0. In the other converter station, active power control at both poles uses constant active power control. In reactive power control, the positive pole uses the station-level reactive power control method of this invention, with the reference value set to the AC node bus voltage, i.e., the PCC voltage setpoint. The negative pole uses constant reactive power control, with the reference value set to 0. The inner-loop controller employs a decoupled control strategy of current feedback and voltage feedforward, converting three-phase AC quantities into two-phase DC quantities in a rotating coordinate system. It features fast current feedback and inherent current limiting capability. The outer-loop controller consists of a steady-state inverse model and a PI regulator, using active or reactive power as the control target to achieve independent regulation of the system's active and reactive power. The block diagram of the active / reactive power decoupling control strategy is shown below. Figure 2 As shown.

[0057] Test Environment: A dual-terminal VSC-MTDC simulation system was built in the PSCAD / EMTDC environment. Converter station 1 and converter station 2 have rated capacities of 2500MW and 4500MW, respectively. For one converter station, two-stage active power control uses constant DC voltage control, and reactive power control uses constant reactive power control, with a reference value set to 0. For the other converter station, two-stage active power control uses constant active power control. In reactive power control, the positive terminal uses the station-level reactive power control method of this invention, with a correction coefficient K of 10 and a reference value set to the AC node bus voltage. The negative terminal uses constant reactive power control with a reference value set to 0. The control strategies for each converter are shown in Table 1. In this example, the reactive power compensation of the converter station accounts for 10% of the total reactive power compensation of the AC system connected to the converter station. Therefore, the reactive power compensation participation coefficient of the converter station is l = 0.1, and the correction coefficient K = 10 in this example.

[0058] Table 1

[0059]

[0060] Simulation results are as follows Figure 4 and Figure 5 , Figure 4 The simulated voltage waveform curve of the DC line at the converter station is shown below. Figure 4 It can be seen that the true bipolar flexible DC transmission system has stability; Figure 5 The voltage signal waveform simulation curve at the converter station's point of common coupling is provided by... Figure 5 It can be seen that the station-level reactive power control method of the present invention can obtain a stable AC voltage.

[0061] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

Claims

1. A station-level reactive power control method for a flexible DC transmission system, characterized in that: include: The measured value U of the voltage signal at the converter station's common coupling point was obtained. s and the measured value of the current signal I s ; Reference value I of reactive component of AC current at converter station common coupling point sqref Measured value of current signal I s q-axis component I sq The difference is compared and taken as the reactive current deviation ΔI at the converter station's point of common coupling. sq ΔI sq The voltage compensation amount ΔU in the AC near-zone of the converter station is obtained after adjusting the gain by a factor of K. ref ; AC voltage reference value U ref With voltage compensation amount ΔU ref and measured AC voltage U s After comparison, the reactive power outer loop deviation ΔU is obtained. After ΔU is adjusted by PI, the reference value I of the reactive power component of the AC current is obtained. sqref The reference value I of the reactive component of the alternating current. sqref As input for inner-loop control; The correction coefficient K = 100l, where l is the reactive power compensation participation coefficient of the converter station, and the value range of l is (0,1]. The voltage compensation amount ΔU in the AC near-zone of the converter station ref The calculation formula is as follows: ΔU ref =K*ΔI sq ; The AC voltage reference value U ref Take the voltage rating U at the converter station's common coupling point set .

2. The station-level reactive power control method for a flexible DC transmission system according to claim 1, characterized in that: The formula for calculating the reactive power outer loop deviation ΔU is as follows: ΔU=U ref -ΔU ref -U s .

3. The station-level reactive power control method for a flexible DC transmission system according to claim 2, characterized in that: The station-level active power control of the flexible DC transmission system adopts constant active power control or constant DC voltage control.

4. The station-level reactive power control method for a flexible DC transmission system according to claim 3, characterized in that: The constant active power control includes the following specific methods: The measured active power P at the converter station's common coupling point was obtained. s The measured value P of the detected active power s With active power reference value P ref The difference is compared and then adjusted by a PI controller to obtain the reference value I for the active component of the AC current. sdref The reference value I of the reactive component of the alternating current. sdref As input for inner-loop control; The constant DC voltage control includes the following specific methods: The measured value U of the DC line voltage of the converter station was obtained. dc The measured value U of the detected DC line voltage dc DC line voltage reference value U dcref The difference is compared and then adjusted by a PI controller to obtain the reference value I for the active component of the AC current. sdref The reference value I of the active component of alternating current. sdref As input for inner loop control.

5. The station-level reactive power control method for a flexible DC transmission system according to claim 4, characterized in that: In the inner loop control, the d-axis component I of the measured current signal is first... sd and the q-axis component I of the measured current signal sq Compared with the reference value I of the active component of alternating current respectively sdref and the reference value I of the reactive component of the alternating current. sqref The differences are compared and then PI-adjusted. The adjusted corrections are then compared with the coupling terms between the d-axis and q-axis currents, as well as the d-axis component U of the measured voltage signal. sd q-axis component U of the measured voltage signal sq The calculations were performed to obtain the d-axis component U of the converter station outlet voltage. cd and the q-axis component of the converter station outlet voltage U cq Then, the d-axis component U of the converter station outlet voltage... cd and the q-axis component of the converter station outlet voltage U cq Input SPWM.

6. The station-level reactive power control method for a flexible DC transmission system according to claim 5, characterized in that: The d-axis component I of the measured current signal sd and the q-axis component I of the measured current signal sq It is by measuring the current signal I s It is obtained by performing a dq transformation.

7. The station-level reactive power control method for a flexible DC transmission system according to claim 6, characterized in that: The d-axis component U of the measured voltage signal value sd q-axis component U of the measured voltage signal sq It is achieved by measuring the voltage signal U at the converter station's common coupling point. s It is obtained by performing a dq transformation.

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