Smooth transition method for active microgrid secondary control

Abstract

The present application relates to a kind of active microgrid secondary control smooth transition method, after microgrid primary control power distribution, when the secondary control of microgrid is regulated to primary control, when the big disturbance beyond set value is encountered in the secondary control startup, shutdown and steady operation of microgrid, smooth transition function is introduced in system frequency and voltage compensation control.The mathematical model of the transition function used in the method is simple and intuitive, only needs to act on the existing secondary control compensation amount by transition function, the smooth transition of secondary control can be realized;Because the action object is the offset compensation amount generated by secondary control, and the existing various secondary controls will generally generate this compensation amount, so it is suitable for any form of secondary control;By using the introduced external independent function, instantaneous impact quantity is weakened and attenuated, while gradually improving and recovering secondary control compensation amount, the big disturbance encountered in the startup, shutdown and steady operation of secondary control can be significantly reduced.

Description

Technical Field

[0001] This invention relates to a hierarchical control technology for microgrids, and more particularly to a smooth transition method for active microgrid secondary control. Background Technology

[0002] Microgrids are considered one of the most effective solutions for large-scale grid integration of distributed generation, including renewable energy sources. A typical microgrid system includes distributed power sources, energy storage, loads, power conversion units, and monitoring and protection devices. Independent, islanded operation is one of the most prominent characteristics of microgrids, often achieved using a hierarchical control method with a three-layer control structure. The lowest layer, primary control, primarily aims to achieve power distribution and frequency / voltage stability within the microgrid system. The second layer, secondary control, mainly restores the frequency / voltage deviation caused by primary control and achieves grid synchronization. The highest layer, tertiary control, focuses on power flow and economic optimization within and outside the microgrid system.

[0003] Currently, secondary control strategies for microgrids can be categorized into three types: centralized, distributed, and discrete. Centralized strategies often employ linear control structures based on proportional-integral controllers, distributed strategies often utilize communication architectures based on graph theory, while discrete strategies typically rely on self-sampling without interconnection communication. However, regardless of the control method, transient offsets and fluctuations inevitably occur when the secondary control system starts up, shuts down, or changes its steady-state operating state. In severe cases, this can even lead to oscillations and loss of control, making the solution to this problem urgent. Existing methods for achieving smooth transitions often rely on global energy management, control coordination, or frequency / voltage synchronization. However, these methods are often difficult to design, lack adaptability, and exhibit significant differences in effectiveness. Summary of the Invention

[0004] To address the issue of smooth transition in microgrid secondary control, an active smooth transition method for microgrid secondary control is proposed. This method incorporates a transition function that directly acts on the secondary control compensation quantity. By weakening and attenuating instantaneous impacts and gradually increasing and restoring the compensation quantity, a smooth transition is achieved. This effectively reduces large disturbances encountered by secondary control during startup, shutdown, and steady-state operation, and avoids secondary control failures, transient oscillations, or accidents such as overvoltage or undervoltage caused by these disturbances.

[0005] The technical solution of this invention is: a smooth transition method for active microgrid secondary control. After the primary control power allocation, when the secondary control of the microgrid regulates the primary control, if a large disturbance encountered during the startup, shutdown, and steady-state operation of the secondary control exceeds a set value, a smooth transition function y is introduced into the system frequency and voltage compensation control. The expression of the smooth transition function y is as follows:

[0006]

[0007] Where 0≤y≤1, x is a transition variable that increases linearly with the control cycle, t is the trigger time, z is a real number greater than 0, and a is the duration.

[0008] Furthermore, the smooth transition control after introducing a smooth transition function in the system frequency and voltage compensation control is as follows:

[0009]

[0010] Among them, f i and E i f represents the output frequency and output voltage of the i-th subsystem, respectively; * and E * These are the system's rated frequency and rated voltage, respectively; m i and n i These are the active and reactive power droop control coefficients for the i-th subsystem, respectively; P i and Q i These represent the active and reactive power outputs of the i-th subsystem, respectively; Δf i and ΔE i These are the frequency compensation and voltage compensation amounts for the i-th subsystem, respectively. An active activation method is adopted, triggering the rising function in the smooth transition function when the secondary control starts and the falling function in the smooth transition function when it stops.

[0011] Furthermore, the microgrid secondary control employs frequency and voltage control based on a distributed consensus algorithm. Frequency control is achieved through the following formula:

[0012]

[0013] Where, Δf i Let u be the frequency compensation amount for the i-th subsystem. fi and u pi Let N be the frequency and active power auxiliary variables of the i-th subsystem, respectively, and 1 / s represent the integration operation over the S-domain; i Let a represent the set of neighboring subsystems of the i-th subsystem. ij f is an element of the adjacency matrix of the i-th subsystem. j f is the output frequency of the j-th neighboring subsystem in the set of neighboring subsystems of the i-th subsystem. i Let p be the output frequency of the i-th subsystem. i d is the restraint coefficient. f For the secondary control frequency coefficient; m j and P j Let d be the droop control coefficient and output active power of the j-th neighboring subsystem in the set of neighboring subsystems of the i-th subsystem. p This refers to the secondary control power coefficient;

[0014] Voltage control is achieved through the following formula:

[0015]

[0016] Where, ΔE i Let u be the voltage compensation amount for the i-th subsystem. Ei and u qi Let be the output voltage and reactive power auxiliary variables of the i-th subsystem, respectively; ω is the cutoff frequency of the reactive power compensation filter. It is the S-domain transfer function of the filter; E j Let d be the output voltage of the j-th neighboring subsystem in the set of neighboring subsystems of the i-th subsystem. v This is the secondary control voltage coefficient.

[0017] The beneficial effects of this invention are as follows: The smooth transition method of active microgrid secondary control is simple and convenient: the mathematical model of the transition function used is simple and intuitive, and the smooth transition of secondary control can be achieved simply by directly applying the transition function to the existing secondary control compensation amount; it has strong applicability: since the object of application is the offset compensation amount generated by the secondary control, and this compensation amount is generally generated by various existing secondary controls, it is applicable to any form of secondary control; the control effect is significant: by using the introduced external independent function to weaken and attenuate the instantaneous impact amount, and at the same time gradually increasing and restoring the secondary control compensation amount, the large disturbances encountered by the secondary control during startup, shutdown and steady-state operation can be significantly reduced. Attached Figure Description

[0018] Figure 1 This diagram shows a comparison of the principles of the smooth transition method for secondary control of microgrids in this invention with traditional methods.

[0019] Figure 2a This is a comparison diagram of the control frequencies of the smooth transition method for microgrid secondary control of the present invention and the traditional method.

[0020] Figure 2b This is a comparison diagram of the control voltage between the smooth transition method for secondary control of microgrids in this invention and the traditional method. Detailed Implementation

[0021] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0022] The specific implementation steps of the smooth transition method for secondary control in this invention are as follows:

[0023] Step 1: For primary control of the microgrid, a power allocation algorithm based on droop control is adopted:

[0024]

[0025] Among them, f i and E i f represents the output frequency and output voltage of the i-th subsystem, respectively. * and E * These represent the system's rated frequency and rated voltage, respectively, m i and n i P represents the active and reactive power droop control coefficients of the i-th subsystem, respectively. i and Q i These are the output active power and reactive power of the i-th subsystem, respectively. Due to the droop factor, the system generates a certain frequency and voltage offset when operating stably.

[0026] Step 2: For the secondary control of the microgrid, frequency and voltage control based on a distributed consensus algorithm are adopted. During stable operation, the system frequency and voltage can recover to near their rated values. Frequency control is achieved through the following formula:

[0027]

[0028] Where, Δf i Let u be the frequency compensation amount for the i-th subsystem. fi and u pi Let N be the frequency and active power auxiliary variables of the i-th subsystem, respectively, and 1 / s represent the integration operation over the S-domain; i Let a represent the set of neighboring subsystems of the i-th subsystem. ij f is an element of the adjacency matrix of the i-th subsystem. j f is the output frequency of the j-th neighboring subsystem in the set of neighboring subsystems of the i-th subsystem. i Let p be the output frequency of the i-th subsystem. i d is the restraint coefficient. f For the secondary control frequency coefficient; m j and P j Let d be the droop control coefficient and output active power of the j-th neighboring subsystem in the set of neighboring subsystems of the i-th subsystem. p This refers to the secondary control power coefficient;

[0029] Voltage control is achieved through the following formula:

[0030]

[0031] Where, ΔE i Let u be the voltage compensation amount for the i-th subsystem. Ei and u qiLet be the output voltage and reactive power auxiliary variables of the i-th subsystem, respectively; ω is the cutoff frequency of the reactive power compensation filter. It is the S-domain transfer function of the filter; E j Let d be the output voltage of the j-th neighboring subsystem in the set of neighboring subsystems of the i-th subsystem. v This refers to the secondary control voltage coefficient;

[0032] Step 3: To address large disturbances exceeding set values ​​(over 0.008 pu for frequency and over 0.025 pu for voltage amplitude) encountered during secondary control startup, shutdown, and steady-state operation, a smooth transition function y is introduced to achieve a smooth transition and reduce transient disturbances and deviations, such as... Figure 1 The diagram shown compares the principles of the smooth transition method for microgrid secondary control of this invention with those of traditional methods. Figure 1 As shown in the left side (a) of the figure, the expression for the smooth transition function y is as follows:

[0033]

[0034] Where 0≤y≤1, x is a transition variable that increases linearly with the control cycle, t is the trigger time, z is a real number greater than 0, and a is the duration;

[0035] Step 4: Use the following formula to achieve a smooth transition:

[0036]

[0037] An active activation method is adopted, which triggers the rising function in the smooth transition function when the secondary control starts and the falling function in the smooth transition function when it stops.

[0038] and Figure 1 Figure (b) on the right shows a traditional microgrid secondary control method. Generally, a distributed consensus secondary control method generates the secondary offset compensation Δf. i and ΔE i It is then directly applied to the power distribution algorithm based on droop control. Compared with the smooth transition method of the secondary control in this invention, it lacks the multiplication of the transition function y.

[0039] A comparison of the control effects of the method of this invention with traditional methods: Figure 2a , 2b As shown, this section examines the initiation of secondary control. The system operates under primary control (step 1) from 0 to 1 second, resulting in a certain frequency and voltage offset in steady state. At 1 second, secondary control (step 2) initiates, i.e., step 3 is executed, corresponding to... Figure 1 The method shown in Figure (a) triggers an ascending function here, with t = 1 and z = 0.3. From... Figure 2a ,2b It can be seen that the method of the present invention significantly reduces the transient offset within 1 to 4 seconds, effectively suppresses the peak value of the impact, achieves a smoother transition, and the system can still reach the rated values ​​of frequency and voltage within 4 to 6 seconds.

[0040] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A smooth transition method for active microgrid secondary control, characterized in that, After the primary control power distribution of the microgrid, when the secondary control of the microgrid regulates the primary control, a smooth transition function is introduced into the system frequency and voltage compensation control when large disturbances encountered during the startup, shutdown, and steady-state operation of the secondary control exceed the set value. y Smooth transition function y The expression is the following formula: , where 0 ≤ y ≤ 1, x It is a transition variable that increases linearly with the control period. t For the trigger time, z For real numbers greater than 0, a The duration is specified; the smooth transition control after introducing a smooth transition function in the system frequency and voltage compensation control is as follows: ,in, f i and E i The first i The output frequency and output voltage of each subsystem; and These are the system's rated frequency and rated voltage, respectively. m i and n i The first i Active and reactive power droop control coefficients for each subsystem; P i and Q i The first i The output active and reactive power of each subsystem; Δ f i and Δ E i The first i The frequency compensation and voltage compensation of each subsystem are calculated. An active activation method is adopted, which triggers the rising function in the smooth transition function when the secondary control starts and the falling function in the smooth transition function when it is turned off.

2. The smooth transition method for active microgrid secondary control according to claim 1, characterized in that, The microgrid secondary control employs frequency and voltage control based on a distributed consensus algorithm. Frequency control is achieved through the following formula: , where Δ f i For the first i Frequency compensation amount of each subsystem u fi and u pi The first i The frequency and active power auxiliary variables of each subsystem, 1 / s express S Integration operations over a field; N i Indicates the first i The set of neighboring subsystems of a subsystem a ij For the first i Adjacency matrix elements of each subsystem f j For the first i The first in the set of adjacent subsystems of the subsystem j The output frequencies of adjacent subsystems f i For the first i The output frequency of each subsystem p i As a restraining coefficient, d f For secondary control frequency coefficients; m j and P j The first i The first in the set of adjacent subsystems of the subsystem j The droop control coefficients and output active power of each adjacent subsystem d p This is the secondary control power coefficient; voltage control is achieved through the following formula: , where Δ E i For the first i Voltage compensation amount of each subsystem u Ei and u qi The first i The output voltage and reactive power auxiliary variables of each subsystem; oh This is the cutoff frequency of the reactive power compensation filter. It is a filter. S Domain transfer function; E j For the first i The first in the set of adjacent subsystems of the subsystem j Output voltage of adjacent subsystems d v This is the secondary control voltage coefficient.

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