Resonant frequency adaptive circuit and method for zero sequence circulating current of converter parallel system
By constructing a low-impedance resonant frequency adaptive path and a controllable switched capacitor circuit in the converter parallel system, the system loss and current distortion caused by low-frequency zero-sequence circulating current are solved, achieving efficient and stable circulating current control, simplifying the control strategy, and improving the system's operating efficiency and reliability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- YANGZHOU XIANGYU ELECTRONICS CO LTD
- Filing Date
- 2025-09-18
- Publication Date
- 2026-07-14
AI Technical Summary
In existing parallel converter systems, low-frequency zero-sequence circulating currents cause increased system losses, current distortion, device overheating, and insulation aging. Furthermore, existing suppression methods suffer from high communication dependence, large computational load, and high parameter sensitivity.
A low-impedance resonant frequency adaptive path is constructed at the neutral point of the filter capacitor. Through a controllable switched capacitor circuit and a PI control loop, the zero-sequence circulating current is actively absorbed and controlled, simplifying the control strategy and reducing the system's dependence on complex algorithms.
It effectively absorbs low-frequency zero-sequence circulating current, reduces system losses, improves system efficiency and stability, avoids current distortion and device overheating, reduces engineering implementation difficulty, and improves system practicality and economy.
Smart Images
Figure CN121150106B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of power electronics technology, specifically to an adaptive circuit and method for the resonant frequency of zero-sequence circulating current in a parallel converter system, and in particular, to absorb the low-frequency zero-sequence circulating current in a parallel converter system within a transformer substation power supply system. Background Technology
[0002] With the support of national policies and the development of the social economy, new energy technologies are rapidly developing, increasing the complexity and power capacity requirements of power distribution systems. To improve system capacity and redundancy, a parallel operation structure of multiple converters is often adopted. Such parallel systems share DC and AC buses, which can expand power levels, but due to differences in hardware parameters, such as filter inductor tolerances, and inconsistent control signals, such as modulation wave phase shift, carrier asymmetry, and asynchronous switching actions, circulating currents can be generated between the parallel converters. Circulating currents can be classified by frequency: high-frequency circulating currents, mainly caused by carrier phase asynchrony or switching timing differences; and low-frequency zero-sequence circulating currents, caused by zero-sequence path voltage imbalance, which are the main components of circulating currents. Low-frequency zero-sequence circulating currents not only increase system losses and lead to decreased efficiency, but also cause current distortion, device overheating, and insulation aging, and in severe cases, damage power devices.
[0003] Currently, the mainstream suppression methods can be divided into two categories: modulation strategy optimization and closed-loop control injection. However, both have their own problems, such as high cost, complex control, and poor suppression effect.
[0004] The core shortcomings of existing technologies include:
[0005] 1) Poor communication dependency and scalability: Schemes such as SVPWM zero vector allocation and closed-loop injection require real-time exchange of zero-sequence voltage data. As the number of parallel units increases, the communication load grows exponentially, the system reliability drops sharply, and communication interruption will lead to uncontrolled loop.
[0006] 2) The contradiction between efficiency and circulating current suppression: SPWM is inefficient; although SVPWM / CBDPWM improves voltage utilization, it causes circulating current due to the inconsistency of zero-sequence components.
[0007] 3) High parameter sensitivity: Factors such as inductance tolerance and temperature drift cause zero-sequence path impedance mismatch, which drastically deteriorates the circulating current suppression effect of the open-loop modulation strategy.
[0008] 4) High computational load: Optimization of control strategies and improvement of modulation methods will increase the computational load of the CPU, resulting in long computation time, which may exacerbate control delay and thus aggravate the circulating current between converters.
[0009] Based on the above shortcomings, there is an urgent need for an adaptive circuit and method for the resonant frequency of zero-sequence circulating current in parallel converter systems that has low communication dependence and simultaneously addresses efficiency and low-frequency circulating current suppression. Its core breakthrough should focus on:
[0010] 1) It has a significant effect on suppressing low-frequency circulating currents;
[0011] 2) Low degree of communication dependence;
[0012] 3) It is easy to control and implement. Summary of the Invention
[0013] To address the shortcomings of existing technologies, this invention discloses an adaptive resonant frequency circuit and method for zero-sequence circulating current in parallel converter systems. This method is simple to implement, has low system dependence, and effectively suppresses low-frequency circulating current in the system. It solves a series of challenging engineering problems caused by low-frequency zero-sequence circulating current in parallel converter systems due to inconsistencies in the amplitude and phase of modulation waves between converters, leading to contradictions between circulating current suppression and overload capacity, large output current distortion and low system efficiency, and conflicts between complex algorithms and engineering practicality.
[0014] To achieve the above objectives, the present invention provides the following technical solution: an adaptive circuit for the resonant frequency of zero-sequence circulating current in a parallel converter system, comprising:
[0015] The output of the three-phase three-bridge-arm grid-connected converter is connected to an AC filter. This structure is used to convert DC power into AC power and connect it to the power grid. The AC filter includes a filter capacitor, and a low-impedance resonant frequency adaptive path is constructed at the neutral point of the filter capacitor.
[0016] The low-impedance resonant frequency adaptive path is connected to the neutral point of the filter capacitor of the AC filter, and this structure is used to guide the zero-sequence circulating current.
[0017] The low-impedance resonant frequency adaptive path includes a controllable switched capacitor circuit and a line inductor. The zero-sequence circulating current first flows through the line inductor and then through the controllable switched capacitor circuit with a variable equivalent capacitance. The controllable switched capacitor circuit includes an H-bridge composed of four MOSFET switches, and the H-bridge is connected in parallel with capacitor C.
[0018] Preferably, the equivalent capacitance value of the capacitor C in the controllable switched capacitor circuit, i.e. the charging and discharging time, is changed by controlling the on and off of the four MOSFET switches in the bridge circuit; and the consistency between the inductor current and the low-frequency zero-sequence circulating current is maintained, thereby controlling the flow of the zero-sequence circulating current.
[0019] Preferably, the on-time of the switching transistor is controlled by a PI control loop. The PI loop has a capacitor voltage regulation structure set using the sign function of the zero-sequence circulating current. The sign function of the zero-sequence circulating current is 1 when the zero-sequence circulating current is in a positive period and -1 when it is in a negative period. The voltage-current relationship of the capacitor is:
[0020]
[0021] This indicates that the current i is a function of the rate of change of the voltage u, where C is the capacitance value;
[0022] Right now
[0023] This means that voltage u is the integral of current i, that is, the accumulation of charge;
[0024] The current i is adjusted by a PI loop, thereby controlling the capacitor voltage u and achieving balanced control of the capacitor voltage.
[0025] Preferably, the voltage U of capacitor C is acquired by a voltage sensor. c With a given value U c-ref The difference is obtained by comparison. The difference is multiplied by the sign function of the zero-sequence circulating current through the set PI control loop and then divided by the magnitude of the capacitor voltage reference value to obtain the sign function modulation wave that controls the capacitor voltage balance.
[0026] Preferably, the inductor current I is collected by a provided current sensor. L Inductor current I L The difference is obtained by comparing the sum of the three-phase AC currents of the main circuit, i.e., the zero-sequence circulating current value. The difference is then output by the PI control loop and superimposed on the sign function modulation wave of the control capacitor voltage equalization to obtain the final modulation wave. The modulation wave is compared with the triangular carrier wave to generate the corresponding PWM wave, which controls the switching transistor to turn on and off, thereby controlling the capacitor voltage stability and the flow of the zero-sequence circulating current.
[0027] Preferably, the capacitor C voltage Uc and zero-sequence circulating current value collected by the voltage sensor and current sensor are used in the PI control loop to control the controllable switched capacitor circuit and control the conduction and cutoff of the four switching transistors in the controllable switched capacitor circuit.
[0028] The present invention also provides a control method for an adaptive circuit of the resonant frequency of zero-sequence circulating current in a parallel converter system, comprising the following steps:
[0029] Step S1: Acquire the voltage U of capacitor C. c The sampled signal is transmitted back to the PI control loop, where it is compared with the preset value U of the program. c-ref Compare the results and obtain the difference.
[0030] The difference obtained in steps S2 and S1 is output through a PI control loop;
[0031] The outputs of steps S3 and S2 are multiplied by the sign functions of the positive and negative poles of the zero-sequence circulating current to obtain the product;
[0032] The product obtained in steps S4 and S3 is divided by the magnitude of the given capacitor voltage to obtain the symbol function modulation wave that controls the capacitor voltage balance.
[0033] Step S5: Collect the current I of inductor L. L The sum of the three-phase AC currents in the main circuit, i.e. the zero-sequence circulating current value, is used to send the sampled signal back to the PI control loop. The two are compared to obtain the difference.
[0034] The difference obtained in steps S6 and S5 is used in the program to obtain a modulation wave that controls the zero-sequence circulating current after passing through a PI control loop.
[0035] Step S7: Superimpose the modulated wave of the control capacitor voltage equalization obtained in step S4 with the modulated wave of the control zero-sequence circulating current obtained in step S6 to obtain the modulated wave signal.
[0036] The modulated wave signals obtained in steps S8 and S7 are compared with the triangular carrier wave to obtain the PWM signal that controls the switching transistor to turn on and off.
[0037] Step S9: Repeat the above steps to form a closed-loop control for the zero-sequence circulating flow of the parallel system.
[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:
[0039] 1. In this invention, an adaptive LC resonant path is constructed at the neutral point of the filter capacitor of the filter. This path becomes a path for mixed-frequency zero-sequence circulating current, allowing it to flow into the DC side midpoint, thereby achieving the purpose of actively absorbing low-frequency circulating current. This effectively absorbs and controls the low-frequency zero-sequence circulating current between converters, which helps to reduce system losses and improve system efficiency. At the same time, it avoids problems such as current distortion, device overheating, and insulation aging caused by circulating current, which helps to extend the service life of converters and other power grid equipment. This is of great significance for improving the operating efficiency of the power grid and reducing energy losses.
[0040] 2. This invention can adapt to fluctuations in grid frequency and the presence of non-primary frequency currents. By introducing closed-loop control of capacitor voltage and inductor current, the voltage of capacitor C can be stabilized, ensuring that the inductor current is consistent with the low-frequency zero-sequence circulating current. This helps improve the stability and reliability of the entire converter parallel system. Even when the grid frequency fluctuates or the circulating current contains complex current frequencies, the resonant adaptive circuit can still complete the flow of low-frequency zero-sequence circulating current and maintain stable system operation.
[0041] 3. This invention simplifies the control strategy for zero-sequence circulating current by designing a controllable switched capacitor circuit. This method not only reduces reliance on complex algorithms but also lowers the difficulty of engineering implementation, making the system easier to deploy and maintain, while improving the system's practicality and economy. Attached Figure Description
[0042] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.
[0043] In the attached diagram:
[0044] Figure 1 This is a schematic diagram of the resonant frequency adaptive circuit for zero-sequence circulating current in the parallel converter system of the present invention; wherein, (a) is a schematic diagram of the system structure, and (b) is a schematic diagram of the controllable switched capacitor structure.
[0045] Figure 2 This is the equivalent circuit and control block diagram of the resonant frequency adaptive circuit for zero-sequence circulating current in the parallel converter system of the present invention.
[0046] Figure 3 This is a schematic diagram of the PI ring structure for capacitor voltage in this invention;
[0047] Figure 4 This is a waveform diagram of the modulation method for generating PWM using a modulation wave in this invention. Detailed Implementation
[0048] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.
[0049] Example: This invention provides an adaptive circuit for the resonant frequency of zero-sequence circulating current in a parallel converter system, such as... Figure 1 As shown in (a), it includes:
[0050] The output terminal of the three-phase three-bridge-arm grid-connected converter is connected to an AC filter. This structure is used to convert DC power into AC power and connect it to the power grid. The AC filter includes a filter capacitor, and a low-impedance resonant frequency adaptive path is constructed at the neutral point of the filter capacitor. The low-impedance resonant frequency adaptive LC resonant path is connected to the neutral point of the filter capacitor of the AC filter. This structure is used to guide the zero-sequence circulating current. The low-impedance resonant frequency adaptive path includes a controllable switched capacitor circuit and a line inductor. The zero-sequence circulating current first flows through the line inductor and then flows through the controllable switched capacitor circuit with a variable equivalent capacitance value. The controllable switched capacitor circuit includes an H-bridge composed of four MOSFET switches. The H-bridge is connected in parallel with capacitor C.
[0051] A simple transformation is performed on the circuit model, as shown in the schematic diagram. Figure 2 As shown. Equivalent circuit description: Equivalent to an H-bridge circuit, the AC side is a current source of mixed-frequency circulating current, and the DC side is capacitor C. During control, it is necessary to stabilize the voltage of capacitor C and ensure that the inductor current matches the low-frequency zero-sequence circulating current. The charging and discharging time of capacitor C and the consistency between the inductor current and the low-frequency zero-sequence circulating current are changed by controlling the on and off states of the four switches in the bridge circuit, thereby controlling the flow of the zero-sequence circulating current. Since the composition of the zero-sequence circulating current is complex, the low-frequency zero-sequence circulating current mainly includes a DC quantity whose amplitude decreases over time and a sinusoidal quantity at odd multiples of the fundamental frequency. Therefore, when adjusting the switching action and conducting current, a certain voltage needs to be maintained across capacitor C. The on-time of the switches is controlled by a PI control loop. Based on the voltage Uc of capacitor C and the zero-sequence circulating current value collected by the voltage and current sensors, the PI control loop controls the controllable switched capacitor circuit, controlling the on and off states of the four switches in the controllable switched capacitor circuit. The voltage U of capacitor C is collected by a voltage sensor. c With a given value U c-ref The difference is obtained by comparison. This difference is then multiplied by the sign function of the zero-sequence circulating current using a pre-defined PI control loop, and divided by the magnitude of the capacitor voltage reference value to obtain the sign function modulation wave for controlling capacitor voltage balance. The inductor current I is then acquired by a pre-defined current sensor. L Inductor current I L The difference is obtained by comparing the sum of the three-phase AC currents of the main circuit, i.e., the zero-sequence circulating current value. The difference is then output by the PI control loop and superimposed on the sign function modulation wave of the control capacitor voltage equalization to obtain the final modulation wave. The modulation wave is compared with the triangular carrier wave to generate the corresponding PWM wave, which controls the switching transistor to turn on and off, thereby controlling the capacitor voltage stability and the flow of the zero-sequence circulating current.
[0052] The PI ring includes a capacitor voltage regulation structure set using the sign function of the zero-sequence circulating current. Specifically, it outputs 1 when the zero-sequence circulating current is in a positive period and -1 in a negative period. The voltage-current relationship of the capacitor is as follows:
[0053]
[0054] This indicates that the current i is a function of the rate of change of the voltage u, where C is the capacitance value;
[0055] Right now
[0056] This means that voltage u is the integral of current i, that is, the accumulation of charge;
[0057] The current i is adjusted by a PI loop, thereby controlling the capacitor voltage u and achieving balanced control of the capacitor voltage.
[0058] The working principle is as follows Figure 2 As shown in the equivalent circuit, when the current flows in the forward direction, as Figure 1 In diagram (b), the arrow points in the direction that controls switches S1 and S4 to conduct, while switches S2 and S3 are turned off. At this time, the current flow path is S1→C→S4. When the current flows in the reverse direction, it is in the same direction as... Figure 1 In diagram (b), the arrows point in opposite directions, turning off control switches S1 and S4 while turning on switches S2 and S3. The current flow path is S3→C→S2. The voltage U across capacitor C is then measured. c The difference is obtained by comparing it with a given value. This difference is then multiplied by the sign function of the zero-sequence circulating current through a PI control loop and divided by the magnitude of the capacitor voltage reference value to obtain the sign function modulation wave for controlling capacitor voltage equalization. The inductor current I is then acquired. L The difference is obtained by comparing the sum of the three-phase AC currents of the main circuit, i.e., the zero-sequence circulating current value. The difference is then output by the PI control loop and superimposed on the sign function modulation wave of the control capacitor voltage equalization to obtain the modulation wave. The modulation wave is compared with the carrier wave to generate the corresponding PWM wave, which controls the switching transistor to turn on and off, thereby controlling the capacitor voltage stability and the flow of the zero-sequence circulating current.
[0059] The present invention also provides a control method for an adaptive circuit of the resonant frequency of zero-sequence circulating current in a parallel converter system, comprising the following steps:
[0060] Step S1: Acquire the voltage U of capacitor C. c The sampled signal is transmitted back to the PI control loop, where it is compared with the preset value U of the program. c-ref Compare the results and obtain the difference.
[0061] The difference obtained in steps S2 and S1 is output through a PI control loop;
[0062] The outputs of steps S3 and S2 are multiplied by the sign functions of the positive and negative poles of the zero-sequence circulating current to obtain the product;
[0063] The product obtained in steps S4 and S3 is divided by the magnitude of the given capacitor voltage to obtain the symbol function modulation wave that controls the capacitor voltage balance.
[0064] Step S5: Collect the current I of inductor L. L The sum of the three-phase AC currents in the main circuit, i.e. the zero-sequence circulating current value, is used to send the sampled signal back to the PI control loop. The two are compared to obtain the difference.
[0065] The difference obtained in steps S6 and S5 is used in the program to obtain a modulation wave that controls the zero-sequence circulating current after passing through a PI control loop.
[0066] Step S7: Superimpose the modulated wave of the control capacitor voltage equalization obtained in step S4 with the modulated wave of the control zero-sequence circulating current obtained in step S6 to obtain the modulated wave signal.
[0067] The modulated wave signals obtained in steps S8 and S7 are compared with the triangular carrier wave to obtain the PWM signal that controls the switching transistor to turn on and off.
[0068] Step S9: Repeat the above steps to form a closed-loop control for the zero-sequence circulating flow of the parallel system.
[0069] When the capacitor voltage is higher or lower than the reference value, such as Figure 3 The PI control loop can control the on-time of the switching transistor to control the rise and fall of the capacitor voltage. However, since the current is an alternating current with different frequencies, it is also necessary to know the positive or negative cycle of the current to control the increase and decrease of the on-time. Therefore, a sign function for the zero-sequence circulating current is added; that is, when the zero-sequence circulating current is in a positive cycle, the output is 1, and when it is in a negative cycle, the output is -1. The sign function of the zero-sequence circulating current polarity is multiplied by the result calculated by the PI control loop to obtain the sign function modulation wave that controls the capacitor voltage balance. The inductor current I is collected. L The difference is obtained by comparing the sum of the three-phase AC currents in the main circuit, i.e., the zero-sequence circulating current value. This difference is then processed by a PI control loop to generate a modulation wave that controls the flow of the zero-sequence circulating current. The sign function modulation wave controlling capacitor voltage equalization is superimposed with the modulation wave controlling the zero-sequence circulating current to generate another modulation wave. This modulation wave is compared with a triangular carrier wave to generate a PWM wave that controls the switching transistor's on / off state. For example, Figure 4 As shown, when the modulated wave is greater than the green carrier wave, switch S1 is turned on, otherwise it is turned off, and S2 is the opposite; when the modulated wave is less than the black carrier wave, switch S3 is turned on, otherwise it is turned off, and S4 is the opposite.
[0070] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. An adaptive circuit for the resonant frequency of zero-sequence circulating current in a parallel converter system, characterized in that, include: The output of the three-phase three-bridge-arm grid-connected converter is connected to an AC filter to convert DC power into AC power and connect it to the power grid. The AC filter includes a filter capacitor, and a low-impedance resonant frequency adaptive path is constructed at the neutral point of the filter capacitor. The low-impedance resonant frequency adaptive path is connected to the neutral point of the filter capacitor of the AC filter to guide the zero-sequence circulating current. The low-impedance resonant frequency adaptive path includes a controllable switched capacitor circuit and a line inductor. The zero-sequence circulating current first flows through the line inductor and then through the controllable switched capacitor circuit with a variable equivalent capacitance. The controllable switched capacitor circuit includes an H-bridge composed of four switching transistors, and the H-bridge is connected in parallel with the capacitor C. The voltage U of capacitor C is collected by a voltage sensor. c With a given value U c-ref The difference is obtained by comparison. The difference is multiplied by the sign function of the zero-sequence circulating current through the set PI control loop and then divided by the magnitude of the capacitor voltage reference value to obtain the sign function modulation wave for controlling capacitor voltage balance. The inductor current I is collected by a current sensor. L Inductor current I L The difference is obtained by comparing the sum of the three-phase AC currents of the main circuit, i.e., the zero-sequence circulating current value, and then the difference is superimposed on the sign function modulation wave of the control capacitor voltage balance after being output by the PI control loop to obtain the final modulation wave. The modulation wave is compared with the triangular carrier wave to generate the corresponding PWM wave, which controls the conduction and turn-off of the switching transistor, thereby controlling the stability of the capacitor voltage and the flow of the zero-sequence circulating current.
2. The resonant frequency adaptive circuit for zero-sequence circulating current in a converter parallel system according to claim 1, characterized in that: The equivalent capacitance value of capacitor C in the controllable switched capacitor circuit is changed by controlling the on and off states of the four switching transistors in the bridge circuit.
3. The adaptive resonant frequency circuit for zero-sequence circulating current in a converter parallel system according to claim 2, characterized in that: The on-time of the switching transistor is controlled by a PI control loop. The PI control loop has a capacitor voltage regulation structure set using the sign function of the zero-sequence circulating current. The voltage-current relationship of the capacitor is as follows: ； Represents current i It is voltage u A function of the rate of change, where C is the capacitance value; Right now ; Indicates voltage u It is electric current i The integral of the charge, i.e., the accumulation of the charge; The current is adjusted by PI control loop. i Thus controlling the capacitor voltage u This enables balanced control of the capacitor voltage.
4. The resonant frequency adaptive circuit for zero-sequence circulating current in a converter parallel system according to claim 3, characterized in that: The capacitor C voltage Uc and zero-sequence circulating current value collected by the voltage sensor and current sensor are used in the PI control loop to control the controllable switched capacitor circuit and control the conduction and cutoff of the four switching transistors in the controllable switched capacitor circuit.
5. A control method for an adaptive resonant frequency circuit of zero-sequence circulating current in a parallel converter system, used in the adaptive resonant frequency circuit of zero-sequence circulating current in a parallel converter system as described in claim 1, characterized in that, Includes the following steps: Step S1: Acquire the voltage U of capacitor C. c The sampled signal is transmitted back to the PI control loop, where it is compared with the preset value U of the program. c-ref Compare the results and obtain the difference. The difference obtained in steps S2 and S1 is output through a PI control loop; The outputs of steps S3 and S2 are multiplied by the sign functions of the positive and negative poles of the zero-sequence circulating current to obtain the product; The product obtained in steps S4 and S3 is divided by the magnitude of the given capacitor voltage to obtain the symbol function modulation wave that controls the capacitor voltage balance. Step S5: Collect the current I of inductor L. L The sum of the three-phase AC currents in the main circuit, i.e. the zero-sequence circulating current value, is used to send the sampled signal back to the PI control loop. The two are compared to obtain the difference. The difference obtained in steps S6 and S5 is used in the program to obtain a modulation wave that controls the zero-sequence circulating current after passing through a PI control loop. Step S7: Superimpose the modulated wave of the control capacitor voltage equalization obtained in step S4 with the modulated wave of the control zero-sequence circulating current obtained in step S6 to obtain the modulated wave signal. The modulated wave signals obtained in steps S8 and S7 are compared with the triangular carrier wave to obtain the PWM signal that controls the switching transistor to turn on and off. Step S9: Repeat the above steps to form a closed-loop control for the zero-sequence circulating flow of the parallel system.