Direct-current short-circuit fault current calculation method suitable for current source converter and suppression strategy of direct-current short-circuit fault current calculation method

Abstract

The invention provides a direct-current short-circuit fault current calculation method suitable for a current source converter and a suppression strategy of the direct-current short-circuit fault current calculation method. At present, a research on a high-voltage direct-current system based on an improved fundamental frequency modulation current source converter (EFFM-CSC) after a direct-current line short-circuit fault occurs is lacked. At present, the research on the high-voltage direct-current system based on the EFFM-CSC after the direct-current line short-circuit fault occurs in the high-voltage direct-current system is short-circuited, and the research on the high-voltage direct-current system based on the EFFM-CSC after the direct-current line short-circuit fault occurs in the high-voltage direct-current system is short-circuited is short-circuited. Aiming at the topology and short circuit characteristics of an EFFM-CSC, according to an additional electromotive force method and an energy storage equivalence principle, a direct current fault equivalent circuit of an improved fundamental frequency modulation CSC-HVDC is established and is subjected to characteristic simplification, a complex structure of a converter is simplified into an RLC equivalent circuit, a direct current fault current calculation formula is deduced according to the direct current fault equivalent circuit, a short circuit transient mathematical model is obtained, and the short circuit transient mathematical model is constructed. The theoretical calculation result is compared with the simulation waveform, and the correctness of the method is verified. For a system direct current short circuit fault current suppression strategy, the invention provides a direct current fault current suppression strategy by utilizing the negative direct current voltage output capability of the EFFM-CSC, and the strategy can quickly clear fault current and has a good fault response characteristic.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission and power electronics, and in particular to a direct current short-circuit fault current calculation method applicable to a current source converter and a suppression strategy thereof. Background Art

[0002] Due to the uneven regional development of energy, high-voltage direct current transmission systems are widely used in power transmission projects due to their advantages such as asynchronous interconnection of power grids, long-distance and large-capacity power transmission, and rapid and controllable power.

[0003] The traditional DC transmission system based on the Line Commutated Converter (LCC) has the advantages of low loss and rich engineering experience. The LCC converter uses semi-controlled thyristors as switching elements, which leads to a low switching frequency of the converter bridge arm and generates a large number of low-order harmonics during operation. In addition, the thyristor needs to provide a reverse voltage from the power grid to restore the shutdown capability, which causes the problem of commutation failure. The Modular Multilevel Converter (MMC) bridge arm adopts a modular design structure and uses the sub-module voltage superposition method to output a step wave AC voltage, which can improve the waveform quality on the AC side. However, compared with thyristors, IGBTs have poor overcurrent tolerance, and the converter station must be equipped with complex protection equipment to achieve DC short-circuit fault crossing.

[0004] The fully controlled current source converter (CSC) adopts the fully controlled device reverse resistance IGCT and new topology, and realizes AC-DC energy conversion by controlling the opening and closing of the bridge arm. The DC side smoothing inductor as an energy storage element can suppress the rise of fault current and has a strong tolerance to short-circuit faults. Pulse width modulation (PWM) and fundamental frequency modulation (FFM), as common modulation methods of CSC at present, have corresponding problems. PWM-CSC has the disadvantages of violent DC voltage fluctuation and large switching loss, which is not conducive to the stable delivery of DC power. FFM-CSC has the advantages of low switching frequency and small voltage fluctuation, but it has only one control degree of freedom and cannot realize independent control of active and reactive power. The improved fundamental frequency modulation current source converter (EFFM-CSC) introduces the compensation angle on the basis of fundamental frequency modulation. Therefore, EFFM-CSC has a trigger angle α and a compensation angle θ, which is more suitable for the field of DC transmission.

[0005] DC line short circuit is one of the main fault types of DC engineering. Short circuit fault analysis provides an important reference for practical engineering design and fault protection. Since the modulation method and steady-state characteristics of EFFM-CSC are different from those of PWM-CSC and FFM-CSC, the DC fault characteristics of CSC-HVDC based on improved fundamental frequency modulation will also be different. Further research is needed on the fault current calculation method and suppression strategy under EFFM-CSC DC short circuit fault. Summary of the invention

[0006] In order to overcome the problems of DC short-circuit fault current calculation and suppression strategy based on current converter, the present invention proposes a DC short-circuit fault current calculation method applicable to current source converter DC, and proposes a corresponding fault current suppression strategy. It is characterized in that the EFFM-CSC short-circuit process is reasonably simplified, and according to the energy storage equivalent principle, the transient mathematical model of the DC system after the fault is derived, and the fault current calculation method is obtained after simplification. In addition, the EFFM-CSC has the ability to output negative DC voltage, and a fault current suppression strategy utilizing the negative DC voltage output capability of the EFFM-CSC is proposed, which can quickly clear the fault current and has good fault response characteristics.

[0007] For the DC transmission system based on EFFM-CSC, when a DC short circuit fault occurs, the potential at the fault location is instantly zero. By superposition principle, the fault response can be decomposed into normal operation and the additional electromotive force E. dc The zero-state response of the fault component circuit under the action of the additional electromotive force is calculated, and the system response under the DC line short circuit is obtained by superimposing it with the system steady-state quantity.

[0008] Based on the energy storage equivalent principle and the valve outlet current expression, the EFFM-CSC converter can be equivalent to maintain the conservation of electromagnetic energy before and after the replacement of this part, using the equivalent inductance L eq , capacitor C eq and resistor R eq By replacing the AC part of the converter station, the equivalent circuit of the fault component of EFFM-CSC can be obtained.

[0009] According to the energy storage equivalent principle, the equivalent inductance L can be obtained eq for:

[0010]

[0011] Where K L is the equivalent coefficient.

[0012] According to the energy storage equivalent principle, the equivalent resistance R eq It is expressed as:

[0013]

[0014] The equivalent inductance L on the AC side can be calculated eq and resistor R eq .

[0015] According to the energy storage equivalent principle, the equivalent capacitance C eq for:

[0016]

[0017] Among them, U dc_0 is the DC voltage output by the CSC before the fault, U c_0 is the peak value of the filter capacitor voltage before the fault.

[0018] According to the above formula, the DC current I of the fault component circuit can be derived dc_f The time domain expression of , the analytical expression of DC current after DC fault occurs is obtained as:

[0019]

[0020] Where: s is the resonant angular frequency of the fault path, I dc_0 is the steady-state value of DC current, ω s By the equivalent inductance L eq and equivalent capacitance C eq Decision, expressed as:

[0021] The present invention also proposes an EFFM-CSC DC fault current suppression strategy. When a DC fault occurs, the system determines that a DC side short circuit fault occurs, I dcN k is the rated current of the DC side during normal operation; h is a margin factor, that is, when the DC current exceeds the rated value k h When the system is short-circuited, the steady-state control strategy is switched to the fault ride-through strategy. For the d-axis link, DC current feedback control is introduced to generate a negative d-axis current command value i during the fault period. rd_ref =-K f i dc , and then control the EFFM-CSC to output a negative DC voltage to achieve rapid clearance of the fault current; for the q-axis link, in order to avoid overvoltage in the AC system caused by DC faults, the constant reactive power control is switched to constant AC voltage amplitude control, and the AC voltage amplitude U rm_ref and the measured value U rm The difference is made, and through the PI link, the q-axis current command value i is generated. rq_ref , and finally achieve rapid suppression of fault current. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a topological structure diagram of the EFFM-CSC direct current transmission system provided by the present invention;

[0023] Figure 2 is an equivalent circuit diagram of the EFFM-CSC fault component provided by the present invention;

[0024] Figure 3 is a comparison diagram of fault current calculation results provided by the present invention;

[0025] Figure 4 It is a block diagram of the DC fault current suppression strategy provided by the present invention; DETAILED DESCRIPTION

[0026] The preferred embodiments are described in detail below in conjunction with the accompanying drawings. It should be emphasized that the following description is only exemplary and is not intended to limit the scope and application of the present invention.

[0027] Figure 1 This is the topological structure diagram of the EFFM-CSC DC transmission system. All converters are analyzed using 12 pulses. Figure 1 As shown, the converter station is composed of two 12-pulse CSCs connected in series, and the sending-end converter station and the receiving-end converter station are connected by a DC transmission line.

[0028] Figure 2 This is the equivalent circuit diagram of the EFFM-CSC fault component. Based on the energy storage equivalent principle and the valve outlet current expression, the EFFM-CSC converter can be equivalent to maintain the conservation of electromagnetic energy before and after the replacement of this part, using the equivalent inductance L eq , capacitor C eq and resistor R eq By replacing the AC part of the converter station, the equivalent circuit of the fault component of EFFM-CSC can be obtained.

[0029] Figure 3 This is a comparison chart of fault current calculation results. Taking the DC line positive pole metallic grounding as an example, the DC current change law on the rectifier side after the DC line fault is analyzed. When a single-pole metallic grounding occurs, the calculation results of the short-circuit current within 10ms after the fault are analyzed. It can be seen from the figure that the fault current calculation results are basically consistent with the detailed model simulation results.

[0030] Figure 4 This is a block diagram of the DC fault current suppression strategy provided by the present invention. When a DC fault occurs, the system determines that a DC side short circuit fault occurs, I dcN k is the rated current of the DC side during normal operation; h is a margin factor, that is, when the DC current exceeds the rated value k hWhen the system is short-circuited, the steady-state control strategy is switched to the fault ride-through strategy. For the d-axis link, DC current feedback control is introduced to generate a negative d-axis current command value i during the fault period. rd_ref =-K f i dc , and then control the EFFM-CSC to output a negative DC voltage to achieve rapid clearance of the fault current; for the q-axis link, in order to avoid overvoltage in the AC system caused by DC faults, the constant reactive power control is switched to constant AC voltage amplitude control, and the AC voltage amplitude U rm_ref and the measured value U rm The difference is made, and through the PI link, the q-axis current command value i is generated. rq_ref , and finally achieve rapid suppression of fault current.

[0031] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Ordinary technicians in the relevant field can still modify or make equivalent substitutions to the specific implementation methods of the present invention with reference to the above embodiments. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims

1. A DC short-circuit fault current calculation method and suppression strategy applicable to a current source converter, characterized in that: The linear circuit superposition theorem is used to decompose the DC fault into normal operation and fault response under additional electromotive force, which simplifies the fault process. The energy storage equivalent principle is used to derive the transient mathematical model of the DC system after the fault and obtain the calculation expression of the fault current.

2. The energy storage equivalence principle according to claim 1, characterized in that: Keeping the conservation of electromagnetic energy before and after replacement, use equivalent inductance, equivalent capacitance, and equivalent resistance to replace the AC part to obtain the equivalent circuit of the fault component.

3. The fault current expression according to claim 1, characterized in that: During the start-up phase of a wind farm, the fault current growth rate is mainly affected by the smoothing reactance, equivalent inductance and equivalent electromotive force, without considering the influence of the filter capacitor.

4. The current source converter according to claim 1, characterized in that: The modulation strategy adopts improved base frequency modulation, which has two control degrees of freedom, allowing independent decoupling control of active and reactive power, and the switching frequency is 150Hz.

5. A DC short-circuit fault current calculation method and suppression strategy applicable to a current source converter, characterized in that: The DC fault ride-through strategy using the negative DC voltage output capability of the EFFM-CSC can quickly clear the fault current and has good fault response characteristics.

6. A DC fault ride-through strategy according to claim 5, characterized in that: Adjust the trigger angle αr and compensation angle θ r is used to control the DC voltage output of EFFM-CSC to a negative voltage to suppress the development of DC fault current.

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