A protection method for flexible DC transmission system based on high frequency component detection

Abstract

The invention discloses a protection method for a flexible direct current transmission system based on high-frequency component detection, and relates to the technical field of flexible direct current transmission. At present, the conventional harmonic detection time is long, and the protection action cannot be quickly activated. The invention first detects the effective value of the fundamental voltage and the effective value of the full-wave voltage of the grid-side voltage, calculates the effective value of the harmonic voltage excluding the fundamental wave through the effective value of the full-wave voltage and the effective value of the fundamental voltage, and then obtains the percentage of harmonic content. The grid-side current is detected, the double frequency component thereof is extracted, and the protection judgment is performed in combination with the fundamental content of the grid-side voltage and the double frequency component content of the grid-side current, and the corresponding grid-side voltage harmonic protection action is executed according to the grid-side voltage harmonic protection scheme, triggering the locking tripping, thereby reducing the influence of the voltage harmonics on the converter and the system, and avoiding the protection malfunction caused by the transformer excitation surge current.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current transmission technology, and in particular to a flexible direct current transmission system protection method based on high frequency component detection. Background Art

[0002] The flexible direct current transmission system adopts switchable power electronic devices, which has the characteristics of flexible control, rapid dynamic response, and low harmonic content. It can quickly and independently adjust active power and reactive power, and has flexible control and coordination capabilities. It is widely regarded as one of the technical means for island power transmission, grid interconnection, and reliable access and effective utilization of new energy, and has broad application prospects.

[0003] Due to the impedance characteristics of the system, the flexible DC transmission system has the possibility of high-frequency oscillation with the AC power grid. When high-frequency oscillation occurs, if the flexible DC system cannot quickly detect the harmonics and perform related locking operations based on the harmonic content, it is very easy to cause damage to the equipment in the converter station or the load equipment related to the AC system. Conventional harmonic detection is based on Fourier decomposition to extract each harmonic in the electrical quantity, but the integer harmonic number extracted after Fourier transform has a large demand on step size and computing power, and the detection time is long, and the protection action cannot be quickly activated. Summary of the invention

[0004] The technical problem to be solved and the technical task proposed by the present invention are to improve and perfect the existing technical solutions and provide a flexible DC transmission system protection method based on high-frequency component detection to achieve rapid protection action and meet the requirements of stable operation of the flexible DC transmission system. To this end, the present invention adopts the following technical solutions.

[0005] A protection method for a flexible direct current transmission system based on high frequency component detection, characterized by comprising the following steps:

[0006] 1) Obtain grid-side voltage and grid-side current sampling values;

[0007] 2) Calculate the full-wave voltage effective value and fundamental voltage effective value based on the grid-side voltage sampling value;

[0008] 3) Calculate the voltage harmonic content ratio U according to the full-wave voltage effective value and the fundamental voltage effective value STHD ;

[0009] 4) Calculate the effective value of the double frequency harmonic current I according to the grid-side current 2RMS ;

[0010] 5) According to the proportion of voltage harmonic content U STHD And the effective value of the second harmonic current I 2RMSThe corresponding grid-side voltage harmonic protection scheme is obtained, and the corresponding grid-side voltage harmonic protection action is executed according to the grid-side voltage harmonic protection scheme to trigger the lockout tripping.

[0011] This technical solution first detects the effective value of the fundamental voltage and the full-wave voltage of the grid-side voltage, and calculates the effective value of the harmonic voltage excluding the fundamental wave through the full-wave voltage effective value and the fundamental voltage effective value, thereby obtaining the harmonic content percentage. The grid-side current is detected, the double frequency component thereof is extracted, and the protection judgment is made in combination with the fundamental content of the grid-side voltage and the double frequency component content of the grid-side current, thereby reducing the impact of voltage harmonics on the converter and the system, and avoiding protection malfunction caused by transformer excitation inrush current.

[0012] This technical solution can quickly protect the system according to the THD content of the high-frequency component, and at the same time, it cannot protect the system against short-term harmonics caused by transformer switching, thereby meeting the requirements for stable operation of the flexible DC transmission system.

[0013] As a preferred technical means: in step 5), the grid-side voltage harmonic protection is divided into three sections;

[0014] The first section: voltage harmonic content ratio U STHD >k set1 After the delay time t1, the corresponding grid-side voltage harmonic protection action is executed, where 50ms≤t1≤120ms, k set1 is a protection setting value, k set1 ≥30%;

[0015] Second paragraph: Voltage harmonic content ratio U STHD >k set2 , delay time t2, and the grid-side current double frequency effective value I 2RMS ≤0.1I 1N When the corresponding grid-side voltage harmonic protection action is executed, 120ms≤t2≤1000ms, k set2 is the second stage protection setting, I 1N is the grid-side current fundamental wave rated value;

[0016] The third section: voltage harmonic content ratio U STHD >k set3 , delay time t3, and the grid-side current double frequency effective value I 2RMS ≤0.1I 1N When the corresponding grid-side voltage harmonic protection action is executed, 1s≤t3≤20s, k set3 It is the three-stage protection setting value, 5%≤k set3 ≤15%; I 1N It is the fundamental wave rated value of the grid-side current.

[0017] Dividing it into three sections helps to strike a balance between efficiency and protection effectiveness.

[0018] As a preferred technical means: the enabling signal of the grid-side voltage harmonic protection action also includes the grid-side voltage fundamental effective value U S1RMS ≤0.9U S1N ; When the grid side voltage fundamental effective value U S1RMS ≤0.9U S1N When the voltage harmonic protection on the grid side is activated, the lockout tripping is triggered.

[0019] As a preferred technical means: in step 5), the control of the grid-side voltage harmonic protection action includes automatic control and manual control, and manual control is performed through the grid-side voltage harmonic second and third stage throw-in and throw-out buttons. Before the transformer is put into operation, the throw-in and throw-out buttons are manually operated to exit the grid-side voltage harmonic second and third stage protection.

[0020] As a preferred technical means: in step 5), the grid-side voltage harmonic protection can be executed only after the converter is unlocked. When the converter is not unlocked, if high-frequency components appear, they are not caused by the flexible DC converter, but may be caused by transformer excitation inrush current, etc., and harmonic protection will not be executed, thereby avoiding false operation.

[0021] Beneficial effects:

[0022] (1) The method for detecting and processing high-frequency components of a flexible direct current transmission system provided by the present invention can quickly detect high-frequency components and avoid damage to equipment caused by high-frequency components;

[0023] (2) The method for detecting and processing high-frequency components of a flexible direct current transmission system provided by the present invention can avoid false operation of high-frequency protection caused by wide-band harmonics when the transformer is put into operation by adding an enabling condition for the effective value of the double-frequency harmonic current. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of the two-terminal flexible direct current transmission system of the present invention;

[0025] Figure 2 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0027] like Figure 1The figure shows a two-terminal flexible direct current transmission system (hereinafter referred to as flexible direct current system), which includes an island side pre-connected AC grid 1, an island side grid-side switch 2, an island side transformer 3, an island side converter valve 4, an overhead line or cable 5, a network side converter valve 6, a network side transformer 7, a network side switch 8, and a network side AC grid 9; the converter valve on one side of the two-terminal flexible direct current transmission system is in island operation, and the converter valve on the other side is in network operation.

[0028] like Figure 2 As shown, the present invention comprises the steps of:

[0029] S1: Obtain grid-side voltage and grid-side current sampling values;

[0030] S2: Calculate the full-wave voltage effective value and the fundamental voltage effective value according to the grid-side voltage sampling value;

[0031] S3: Calculate the voltage harmonic content proportion U based on the full-wave voltage effective value and the fundamental voltage effective value STHD ;

[0032] The grid-side voltage harmonic protection is divided into three sections:

[0033] The first section: voltage harmonic content ratio U STHD >k set1 After the delay time t1, the protection action, where 50ms≤t1≤120ms, k set1 is a protection setting value, k set1 ≥30%;

[0034] Second paragraph: Voltage harmonic content ratio U STHD >k set2 , delay time t2, and the grid-side current double frequency effective value I 2RMS ≤0.1I 1N When the corresponding grid-side voltage harmonic protection action is executed, 120ms≤t2≤1000ms, k set2 The second stage protection setting value, 15% ≤ k set1 ≤30%; and the grid-side current double frequency effective value I 2RMS ≤0.1I 1N When the second stage protection is enabled, I 1N is the grid-side current fundamental wave rated value;

[0035] The third section: voltage harmonic content ratio U STHD >k set3 , delay time t3, and the grid-side current double frequency effective value I 2RMS ≤0.1I 1N When the corresponding grid-side voltage harmonic protection action is executed, 1s≤t3≤20s, k set3 It is the three-stage protection setting value, 5%≤k set3≤15%; I 1N It is the fundamental wave rated value of the grid-side current.

[0036] S4: Calculate the effective value of the double frequency harmonic current I according to the grid-side current 2RMS ;

[0037] S5: According to the proportion of voltage harmonic content U STHD And the effective value of the second harmonic current I 2RMS The corresponding grid-side voltage harmonic protection scheme is obtained, and the corresponding grid-side voltage harmonic protection action is executed according to the grid-side voltage harmonic protection scheme to trigger the lockout tripping.

[0038] In order to better protect the flexible DC transmission system, the enabling signal of the grid-side voltage harmonic protection action also includes the grid-side voltage fundamental effective value U S1RMS ≤0.9U S1N ; When the grid side voltage fundamental effective value U S1RMS ≤0.9U S1N When the voltage harmonic protection on the grid side is activated, the lockout tripping is triggered.

[0039] To improve the flexibility of control, in step 5), the control of the grid-side voltage harmonic protection action includes automatic control and manual control. Manual control is performed through the grid-side voltage harmonic second-stage and third-stage throw-in and throw-out buttons. Before the transformer is put into operation, the throw-in and throw-out buttons are manually operated to exit the grid-side voltage harmonic second-stage and third-stage protections.

[0040] In order to avoid false protection and improve the stability of operation, in step 5), the grid-side voltage harmonic protection can only be executed after the converter is unlocked.

[0041] The flexible DC transmission system protection method based on high-frequency component detection shown above is a specific embodiment of the present invention, which has embodied the substantial characteristics and progress of the present invention. According to actual use needs and under the guidance of the present invention, equivalent modifications in shape, structure, etc. can be made to the method, which are all within the protection scope of this scheme.

Claims

1. A protection method for a flexible DC transmission system based on high frequency component detection, characterized in that The following steps are involved: 1) Obtain grid-side voltage and grid-side current sampling values; 2) Calculate the full-wave voltage effective value and fundamental voltage effective value based on the grid-side voltage sampling value; 3) Calculate the proportion of voltage harmonic content based on the full-wave voltage effective value and the fundamental voltage effective value ; 4) Calculate the effective value of the double frequency harmonic current based on the grid-side current ; 5) According to the proportion of voltage harmonic content and the effective value of the second harmonic current Obtain the corresponding grid-side voltage harmonic protection scheme, and execute the corresponding grid-side voltage harmonic protection action according to the grid-side voltage harmonic protection scheme to trigger the lockout trip; In step 5), the grid-side voltage harmonic protection is divided into three sections; The first paragraph: voltage harmonic content ratio , after the delay time t1, the corresponding grid-side voltage harmonic protection action is executed, where , is a protection setting value. ; Second paragraph: voltage harmonic content ratio , delay time t2, and the grid-side current double frequency effective value When the corresponding grid-side voltage harmonic protection action is executed, , is the second stage protection setting value, ; is the grid-side current fundamental wave rated value; The third paragraph: voltage harmonic content ratio , delay time t3, and the grid-side current double frequency effective value When the corresponding grid-side voltage harmonic protection action is executed, , It is the three-stage protection setting value. ; is the fundamental wave rated value of the grid-side current; The enabling signal of the grid-side voltage harmonic protection action also includes the grid-side voltage fundamental effective value ; When the grid side voltage fundamental effective value When the grid-side voltage harmonic protection is activated, the lockout trip is triggered; In step 5), the control of the grid-side voltage harmonic protection action includes automatic control and manual control. Manual control is performed through the grid-side voltage harmonic second-stage and third-stage throw-in and throw-out buttons. Before the transformer is put into operation, the throw-in and throw-out buttons are manually operated to exit the grid-side voltage harmonic second-stage and third-stage protections. In step 5), the grid-side voltage harmonic protection can be performed only after the converter is unlocked.

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