CN105140968A - Microgrid transient performance intensification device and method based on cooperative control of fault current limit and quick energy storage - Google Patents

Abstract

The present invention relates to a micro-grid transient performance enhancement device and method based on fault current limiting-fast energy storage coordinated control, as follows: when the micro-grid encounters an external short-circuit fault, considering the difference in fault location and the interactive demand for high penetration power, Under certain fault conditions, the coordinated control of the current limiter and the fast energy storage device can improve the fault ride-through capability of the microgrid; for some specific conditions such as the short-circuit fault of the tie line of the microdistribution network, the microgrid must be disconnected from the main network At the same time, the coordinated control of the fault current limiter and the fast energy storage device can ensure the smooth transition of the microgrid between the grid connection and the island mode. The present invention can effectively limit the fault current and reduce the voltage drop at the coupling point of the micro-grid, and also play an active role in stabilizing the power fluctuation of the micro-grid and improving the power quality, thereby improving the ability of the high-permeability micro-grid to deal with external short-circuit faults, guarantee its operational reliability.

Description

A microgrid transient performance enhancement device and method based on fault current limiting-fast energy storage coordinated control

technical field

The invention belongs to the field of electric power system and automation, and in particular relates to a micro-grid transient performance enhancement device and method based on fault current limiting-fast energy storage coordinated control, which is used to improve the robustness of high-permeability micro-grid against external short-circuit faults , to ensure its transient performance.

Background technique

Under the dual pressure of energy demand and environmental protection, both at home and abroad are focusing on the research field of distributed generation (DG) of renewable energy, and building a sustainable energy system has become the consensus and inevitable development trend of all countries. Existing research and practice results show that wind power generation, solar photovoltaic, fuel cell and other DGs connected to energy storage devices and loads to form a micro-grid operation can give full play to the efficiency and advantages of renewable energy power generation, and contribute to the important role of the power grid in the event of catastrophe. The continuous and stable power supply of load is of great significance to realize the flexible, stable and efficient application of distributed power generation technology on the medium and low voltage level.

In fact, when the microgrid is connected to the main network in the form of high penetration rate (usually means that the proportion of exchanged power exceeds 10% of the distribution network capacity), the microgrid will have an important impact on the operating characteristics of the connected distribution network. , especially when a short-circuit fault occurs, the coping mechanism and transient performance of the high-permeability microgrid will have an important impact on the operation of the distribution network. How to ensure the safe and reliable operation of the high-permeability microgrid and the distribution network under fault conditions is an urgent need One of the key problems to be solved.

When a short-circuit fault occurs outside the microgrid, whether the microgrid should be operated as a fault ride-through or an island operation, there is no clear treatment strategy and solution in both academia and engineering practice. The IEEE1547 standard stipulates the exit conditions for distributed power generation to deal with main grid failures, but after the distributed power generation is connected to the grid in the form of a microgrid, the situation is quite different. If any external fault triggers the switch on the tie line of the common coupling point to isolate the microgrid, it will have a negative impact on the reliability and flexibility of the operation of the microgrid, and in a sense it does not meet the original intention of building a microgrid .

Fault current limiter (fault current limiter, FCL) is an auxiliary device commonly used to improve the power grid system and its power equipment's ability to deal with faults. When FCL is applied in a distribution network containing a high-permeability micro-grid, there are certain requirements for the fault current limit rate. Different from the large power grid system, it is necessary to reduce the short-circuit current by 10 to 20 times under the interrupting capacity of the circuit breaker. In addition, the microgrid is usually equipped with an energy storage device, and its basic purposes include: providing short-term power supply, power peak regulation, improving the performance of the micropower supply, etc. Strong efficacy. When the fault current limiting technology is applied to the high penetration microgrid, it can be further coordinated with the energy storage system for control. By integrating the technical advantages of current limiting and energy storage, and taking into account the strengths of the two to formulate a matching coordinated action strategy, it is beneficial to better improve the power supply reliability and operation of the high-penetration microgrid and distribution network under fault conditions. stability.

The invention relates to a micro-grid transient performance enhancement method based on fault current limiting-fast energy storage coordinated control, in order to limit the fault current and reduce the voltage drop at the coupling point, while also stabilizing power fluctuations and improving power quality. to a positive effect. It enables the microgrid to achieve fault ride-through because the transient performance meets the technical requirements when there is no need for island operation; when the microgrid should be converted to island operation, the coordination of current limiting and energy storage helps to reduce the static switching action at the public coupling point. shock oscillation, and realize the smooth transient transition between microgrid grid-connected/island operation modes.

Contents of the invention

The present invention aims at a high-permeability micro-grid system containing multiple distributed generation (DG) units, integrates and utilizes a fault current limiter and a fast energy storage device, and proposes a coordinated control method to enhance the high-permeability micro-grid to cope with external short-circuit faults robustness.

The present invention adopts following technical scheme to realize:

A microgrid transient performance enhancement device based on fault current limiting-fast energy storage coordinated control, characterized in that a fault current limiter and a fast energy storage device are installed at the public coupling point between the microgrid and the main grid; wherein,

The fault current limiter is a magnetic flux coupling superconducting fault current limiter, which is composed of a coupling transformer, a fast controllable switch S ₁ , a high temperature superconducting resistor and an MOA zinc oxide resistor. The primary coil L ₁ of the coupling transformer is connected with the fast switch and incorporated into the MOA, the secondary coil L ₂ is connected in series with the superconducting resistor, and then the primary and secondary coils are connected in reverse parallel to the public grid between the microgrid and the main grid. at the coupling point. When the power grid is running normally, the fast switch is in the closed state, the coupling transformer shows a non-inductive effect, and the superconducting resistance represents a zero-resistance effect, and the fault current limiter has no effect on the operation of the power grid; after a short-circuit fault occurs, the controllable switch is quickly disconnected , the primary and secondary coils of the coupling transformer are decoupled, and the superconducting resistor presents a high-impedance state due to the current exceeding its critical value. At this time, the current limiter is connected in series to the main circuit to suppress the rise of the fault current and improve the voltage at the common coupling point. fall.

The selected fast energy storage is a superconducting magnetic energy storage device. The superconducting magnetic energy storage device is a coil made of superconducting material. It is excited by the power grid through a converter, and a magnetic field is generated in the coil to store energy. This energy can be fed back to the grid through the inverter; if the energy storage coil is always maintained in a superconducting state, the energy stored in the coil can be stored permanently with almost no loss until it needs to be released; compared with other energy storage systems , the superconducting magnetic energy storage device has high conversion efficiency (95%) and fast response speed (millisecond level); the superconducting coil is connected to the fully-controlled converter after being processed by the DC power supply device, and then connected to the microgrid system; the DC power processing device has two basic operating modes, magnetizing mode and demagnetizing mode, which correspond to the charging state and discharging state of the energy storage system respectively; in one switching cycle, the average power of the energy storage system is determined by the processing device The duty cycle (D) of the power electronic switches S1 and S2 is controlled by the adjustment; when D>0.5, the energy storage system will absorb energy, and when D<0.5, it will release energy; the duty cycle can be determined by the following formula calculation:

P _SMES ＝u _C ·(2D-1)·i _SMES (1)

A microgrid transient performance enhancement method based on fault current limiting-fast energy storage coordinated control, characterized in that: the specific method is:

When the power grid is operating normally, the fault current limiter presents low impedance and has no effect on the microgrid and the connected distribution network. The fast energy storage device uses an active-reactive power control strategy to stabilize the power fluctuation inside the microgrid. When the microgrid encounters an external short-circuit fault, the coordinated control of the current limiter and the fast energy storage device under certain fault conditions can improve the fault ride-through capability of the microgrid; When the public coupling connection line is short-circuited and the microgrid must be disconnected from the main network, the current limiter will suppress the fault current rise and improve the voltage drop, and at the same time send the action signal to the fast energy storage device, so that the energy storage device switches to the droop control mode , to stabilize the frequency and voltage of the microgrid, and realize the smooth transition of the microgrid between grid-connected and island modes, specifically including the following selection steps:

Selection step 1: When the short-circuit fault location is downstream of the power distribution system connected to the microgrid, the dominant factor that determines the fault ride-through or island operation of the microgrid at this time should be: the amount of power exchange between the microgrid and the main system. As far as the high-penetration microgrid is concerned, starting from improving the utilization rate of renewable energy, taking into account its power support to the distribution network in the event of a fault, and helping to avoid further expansion of the grid power deficit, assisting this type of microgrid to realize Fault ride-through is beneficial to the safe and reliable operation of the system. Therefore, the purpose of applying the current limiter and the energy storage device is to improve the fault ride-through capability of the microgrid. Installing fault current limiters can help limit fault currents caused by the microgrid and at the same time compensate for voltage dips at points of common coupling. When the microgrid is still in the networking mode, the energy storage system acts to stabilize the power fluctuations in the microgrid, that is, the active-reactive power control method is adopted. Among them, the double closed-loop power controller is used to generate the reference value of the current signal, so as to realize the fast tracking of active and reactive power. The mathematical equation can be expressed as:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; \&Integral;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; q</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; \&Integral;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Among them, K _PV and K _iv are the proportional and integral coefficients of the voltage controller; V ^* _ld and V ^* _lq represent the d-axis and q-axis reference signals of the DC voltage in the DC power processing device.

Selection step 2: When the short-circuit fault occurs at the tie line at the public coupling point or at the upstream of the power distribution system connected to the microgrid, the relay protection device configured on the tie line will trip the static switch after several power frequency cycles , the microgrid must be separated from the main grid. While suppressing the fault current rise and improving the voltage drop, the current limiter transmits the action signal to the fast energy storage device, so that the energy storage device switches to the droop control mode, so as to stabilize the frequency and voltage of the microgrid, and realize the grid connection of the microgrid. Smooth transition between and island mode.

After the current limiter operates quickly, the short-circuit current can be reasonably suppressed, and the voltage drop at the common coupling point can be effectively improved, which is positive for the smooth transition of the microgrid to the isolated grid state. Then, the static switching action makes the microgrid off-grid, and the energy storage system switches from active and reactive power control to droop control during grid connection, so as to stabilize the voltage and frequency of the microgrid. The reference voltage E ^* and the reference frequency ω ^* of the common coupling point can be obtained by the following formula:

ω ^* ＝ω ₀ -K _ω P(4)

E ^* ＝E ₀ -K _E Q(5)

Among them, K _ω and K _E are the proportional coefficients of the frequency controller and voltage controller, respectively; P and Q represent the active and reactive power of the fast energy storage system.

A micro-grid transient performance enhancement method based on fault current limiting-fast energy storage coordinated control involved in the present invention has the following advantages: reduce the short-circuit current on the tie line of the public coupling point, and improve the voltage drop of the public coupling point , reduce the power fluctuation (shortage) inside the microgrid, suppress the amplitude of its frequency oscillation, and improve the power quality level of the microgrid. The invention helps to more clearly define the basic principles and practical criteria of microgrid fault isolation under different short-circuit conditions, and provides a practical technical solution to improve the temporary efficiency of microgrid during fault ride-through and switching to island operation. It is of great significance to ensure the safe and reliable operation of the high-permeability microgrid and its connected distribution network.

Description of drawings

Accompanying drawing 1 is the structural diagram of the microgrid in the example application of the present invention.

Accompanying drawing 2 is the structure diagram of the fault current limiter selected in the example application of the present invention.

Accompanying drawing 3 is the schematic structural diagram of the fast energy storage device selected in the example application of the present invention.

Accompanying drawing 4 is the control block diagram (grid-connected mode) of the fast energy storage device in the example application of the present invention.

Accompanying drawing 5 is the control block diagram (island mode) of fast energy storage device in the example application of the present invention.

Accompanying drawing 6 is the electrical characteristic (normal operating condition) at the public coupling point of the microgrid in the example application of the present invention.

Accompanying drawing 7 is the electrical characteristics at the public coupling point of the microgrid in the example application of the present invention (F1 point fault grounding).

Accompanying drawing 8 is the load power characteristic of the microgrid and the output power waveform of the fast energy storage device in the example application of the present invention (F1 point fault grounding)

Accompanying drawing 9 is the switching power and frequency fluctuation waveform of the microgrid in the example application of the present invention (fault grounding at point F1).

Accompanying drawing 10 is the reactive power waveform of the micro-grid in the example application of the present invention (fault grounding at point F1).

Accompanying drawing 11 is the electrical characteristics at the public coupling point of the microgrid in the example application of the present invention (fault grounding at point F2).

Accompanying drawing 12 is the load power characteristics of the microgrid and the output power waveform of the fast energy storage device in the example application of the present invention (fault grounding at point F2).

Accompanying drawing 13 is the switching power and frequency fluctuation waveform of the microgrid in the example application of the present invention (fault grounding at point F2).

Accompanying drawing 14 is the reactive power waveform of the micro-grid in the example application of the present invention (fault grounding at point F2).

Detailed ways

As shown in FIG. 1 , it is a schematic structural diagram of a microgrid in an example application of the present invention. The selected microgrid contains DG units such as photovoltaic power generation and wind power generation, and each DG unit is connected to the microgrid system through an inverter device. Regarding the specific implementation of the current limiting-energy storage coordinated control method, the coordinated control involved in the present invention is described in detail by taking the magnetic flux coupling type superconducting fault current limiter and the superconducting magnetic energy storage device as the specific implementation objects. method.

As shown in Fig. 2, it is the structural representation of the selected fault current limiter in the example application of the present invention, this type current limiter is the magnetic flux coupling type superconducting fault current limiter (superconducting fault current limiter, SFCL), by coupling transformer, fast controllable Composed of switch S ₁ , high temperature superconducting resistors and MOA zinc oxide resistors. The primary coil L ₁ of the coupling transformer is connected with the fast switch and incorporated into the MOA, the secondary coil L ₂ is connected in series with the superconducting resistor, and then the primary and secondary coils are connected in reverse parallel to the public grid between the microgrid and the main grid. Coupling point (pointofcommoncoupling, PCC). When the power grid is running normally, the fast switch is in the closed state, the coupling transformer shows a non-inductive effect, and the superconducting resistance represents a zero-resistance effect, and the fault current limiter has no effect on the operation of the power grid; after a short-circuit fault occurs, the controllable switch is quickly disconnected , the primary and secondary coils of the coupling transformer are decoupled, and the superconducting resistor presents a high-impedance state due to the current exceeding its critical value. At this time, the current limiter is connected in series to the main circuit to suppress the rise of the fault current and improve the voltage at the common coupling point. fall.

As shown in FIG. 3 , it is a schematic structural diagram of a selected fast energy storage device in an example application of the present invention, and the selected fast energy storage device is a superconducting magnetic energy storage device (SMES). The superconducting magnetic energy storage device is a coil made of superconducting material, which is powered and excited by the power grid through a converter, and a magnetic field is generated in the coil to store energy. When needed, this energy can be fed back to the power grid through an inverter. If the energy storage coil is maintained in a superconducting state, the energy stored in the coil can be stored permanently with almost no loss until it needs to be released. Compared with other energy storage systems, superconducting magnetic energy storage devices have higher conversion efficiency (95%) and faster response speed (millisecond level). The superconducting coil in this example application is connected to the fully controlled converter after the chopper, and then connected to the microgrid system. The chopper has two basic modes of operation, the magnetization mode and the magnetization mode, which correspond to the state of charge and discharge of the energy storage system, respectively. During one switching cycle, the average power of the energy storage system is controlled by the regulation of the duty cycle (D) of the power electronic switches S1 and S2. When D>0.5, the energy storage system will absorb energy, and when D<0.5, it will release energy. The duty cycle can be calculated by the following formula:

P _SMES ＝u _C ·(2D-1)·i _SMES (1)

For the control strategy of the energy storage system, it is directly related to the operating state of the microgrid. When the short-circuit fault location is point F1 shown in the figure, the dominant factor determining microgrid fault ride-through or island operation should be: the amount of power exchange between the microgrid and the main system. As far as the high-penetration microgrid is concerned, starting from improving the utilization rate of renewable energy, taking into account its power support to the distribution network in the event of a fault, and helping to avoid further expansion of the grid power deficit, assisting this type of microgrid to realize Fault ride-through is beneficial to the safe and reliable operation of the system. Therefore, the purpose of applying the current limiter and the energy storage device is to improve the fault ride-through capability of the microgrid. Installing SFCLs can help limit fault currents caused by microgrids and at the same time compensate for voltage dips at points of common coupling. When the microgrid is still in the networking mode, the energy storage system acts to stabilize the power fluctuations in the microgrid, that is, the active-reactive power control method is adopted, as shown in Figure 4. Among them, the double closed-loop power controller is used to generate the reference value of the current signal, so as to realize the fast tracking of active and reactive power. The mathematical equation can be expressed as:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; \&Integral;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; q</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; \&Integral;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; q</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Among them, K _PV and K _iv are the proportional and integral coefficients of the voltage controller; V ^* _ld and V ^* _lq represent the d-axis and q-axis reference signals of the DC capacitor voltage in the chopper.

When a short-circuit fault occurs at point F2, the relay protection device configured on the tie line at the public coupling point will trip the static switch after a few power frequency cycles, and the microgrid must be separated from the main grid. While suppressing the fault current rise and improving the voltage drop, the current limiter transmits the action signal to the fast energy storage device, so that the energy storage device switches to the droop control mode, so as to stabilize the frequency and voltage of the microgrid, and realize the grid connection of the microgrid. Smooth transition between and island mode. The control block diagram of the energy storage system under this working condition is shown in Fig. 5.

After the current limiter operates quickly, the short-circuit current can be reasonably suppressed, and the voltage drop at the common coupling point can be effectively improved, which is positive for the smooth transition of the microgrid to the isolated grid state. Then, the static switching action makes the microgrid off-grid, and the energy storage system switches from active and reactive power control to droop control during grid connection, so as to stabilize the voltage and frequency of the microgrid. The reference voltage E ^* and the reference frequency ω ^* of the common coupling point can be obtained by the following formula:

ω ^* ＝ω ₀ -K _ω P(4)

E ^* ＝E ₀ -K _E Q(5)

Among them, K _ω and K _E are the proportional coefficients of the frequency controller and voltage controller, respectively; P and Q represent the active and reactive power of the fast energy storage system.

In order to verify the specific performance of the coordinated control method proposed in the present invention, a detailed electromagnetic transient simulation model is established with reference to Figure 1, and its simulation parameters are shown in the following table.

The system simulation parameters in the specific implementation of the attached table invention

Figure 6 shows the power characteristics of the DG unit in normal operation of the power grid. At this time, the current limiter has no effect on the operation of the grid. The fast energy storage system is used to stabilize the power fluctuations caused by wind farms and photovoltaic power generation. The overall active power of the distributed units will be controlled to 250 kW (P _PV +P _wind +P _SMES =250kW). That is, the load gap of 270kW inside the microgrid will be provided by the main grid.

Assume that the three-phase ground fault occurs at point F1, and the duration is 200ms. Before the short circuit fault is cleared by the relay protection device, the voltage sag at the common coupling point will directly affect the power exchange between the main grid and the micro grid. In fact, after a fault, the power interaction between the two will be reversed, and the microgrid will deliver energy to the main grid. In view of the fact that the microgrid still maintains active and reactive power control in the grid-connected mode (considering fault ride-through), the simulation analysis only considers whether to install a current limiter. Figure 7-10 shows the operating characteristics of the microgrid Detailed waveform. From the simulation results, the introduction of the fault current limiter helps to reduce the fault current flowing from the microgrid, improve the voltage drop of the common coupling point and suppress the frequency/reactive power fluctuation of the microgrid, which is helpful for improving the fault ride-through of the microgrid. Ability will have a positive effect. Here is a brief description of the detailed operating data: the application of the current limiter can limit the fault current from 95A to 68A, and the current limiting rate is 28.4%; The fluctuation amplitude can be suppressed within 0.21Hz.

When the three-phase ground fault occurs at point F2, the static switch of the public coupling point will trip 140ms after the fault, causing the microgrid to go off-grid, and then after 0.5 seconds, the reclosing operation will make the microgrid reconnect to the grid. Under this fault condition, a total of three operating scenarios are simulated, corresponding to: 1) no fault current limiter is installed; 2) the fault current limiter is installed but its coordination with the energy storage device is not considered; 3) The current limiting-energy storage coordinated control method proposed by the present invention is introduced. In addition, when the microgrid is converted to island operation, in order to further ensure its power quality, the load shedding operation is also properly considered (for load 1 and load 3).

Figure 11-14 shows the operating characteristics of the microgrid when F2 is faulted to ground. It can be concluded that through coordinated control of the fault current limiter and energy storage system, it will play a more effective and comprehensive role in suppressing fault current, improving voltage drop, stabilizing system frequency and reducing power fluctuations. effect. Taking the enhancement of system frequency stability as an example, coordinated control can limit the frequency fluctuation within 0.28Hz. Combined with the effective limitation of the fault current and the improvement of the voltage sag at the point of common coupling, the microgrid system can transition to the island mode smoothly and smoothly.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, but they will not deviate from the spirit of the present invention or go beyond the definition of the appended claims range.

Claims (2)

1. A micro-grid transient performance strengthening device based on fault current limiting-rapid energy storage coordination control is characterized in that a fault current limiter and a rapid energy storage device are installed at a public coupling point of a micro-grid and a main grid; wherein,

the fault current limiter is a magnetic flux coupling type superconducting fault current limiter and consists of a coupling transformer and a rapid controllable switch S₁The high-temperature superconducting resistor and the MOA zinc oxide resistor disc; primary coil L of coupling transformer₁Associated with a fast switch and incorporating MOA, secondary winding L₂Is connected in series with a superconducting resistorConnecting, namely reversely connecting the primary and secondary side winding coils into a public coupling point of the microgrid and the main grid; when the power grid normally operates, the quick switch is in a closed state, the coupling transformer presents a non-inductive effect, the superconducting resistance represents a zero resistance effect, and the fault current limiter has no influence on the operation of the power grid; after short-circuit fault occurs, the controllable switch is quickly switched off, the primary coil and the secondary coil of the coupling transformer are decoupled, the superconducting resistor is in a high-resistance state because the current exceeds the critical value of the superconducting resistor, and the current limiter is connected in series to the main loop to inhibit the rise of fault current and improve the voltage drop at the common coupling point;

the selected fast energy storage is a superconducting magnetic energy storage device, the superconducting magnetic energy storage device is a coil made of superconducting materials, a power grid supplies power and excites through a converter, a magnetic field is generated in the coil to store energy, and the energy can be fed back to the power grid through an inverter when needed; if the storage coil is maintained in a superconducting state, the energy stored in the coil can be permanently stored almost without loss until release is required; compared with other energy storage systems, the superconducting magnetic energy storage device has higher conversion efficiency and faster reaction speed; the superconducting coil is processed by a direct current power supply device and then connected to a full-control converter, and then connected to a micro-grid system; the direct-current power supply processing device has two basic operation modes, namely a magnetizing mode and a demagnetizing mode, which respectively correspond to the charging state and the discharging state of the energy storage system; the average power of the energy storage system is controlled by the adjustment of the duty cycle D of the power electronic switches S1 and S2 in the processing device during one switching cycle; when D >0.5, the energy storage system will absorb energy, when D <0.5, it will release energy; the duty cycle may be calculated by:

P_SMES＝u_C·(2D-1)·i_SMESthe formula I is shown.

2. A micro-grid transient performance strengthening method based on fault current limiting-rapid energy storage coordination control is characterized by comprising the following steps: the specific method comprises the following steps:

when the power grid normally operates, the fault current limiter presents low impedance and has no influence on the microgrid and a connected power distribution network, and the rapid energy storage device uses an active-reactive power control strategy to stabilize power fluctuation inside the microgrid; when the micro-grid suffers from external short-circuit faults, the current limiter and the rapid energy storage device are coordinately controlled under certain fault working conditions to improve the fault ride-through capability of the micro-grid; for fault conditions of some characteristic positions, such as a microgrid interconnection line short circuit, when a microgrid has to be disconnected from a main network, a current limiter inhibits the rise of fault current and improves the voltage drop, and simultaneously transmits an action signal to a rapid energy storage device, so that the energy storage device is switched to a droop control mode, the frequency and the voltage of the microgrid are stabilized, and the smooth transition of the microgrid between a grid-connected mode and an island mode is realized, and the method specifically comprises the following selection steps:

selecting the step 1: when the short-circuit fault position is at the downstream of the distribution system connected with the microgrid, the leading factors determining the fault crossing or island operation of the microgrid are as follows: the amount of power exchange between the microgrid and the main system; for a high-permeability microgrid, on the basis of improving the utilization rate of renewable energy sources, the power supply support of the microgrid on a power distribution network under the condition of a fault is taken into consideration, the power shortage of the microgrid can be further favorably avoided, and the microgrid is assisted to realize fault ride-through, so that the microgrid is beneficial to safe and reliable operation of a system; therefore, the purpose of applying the current limiter and the energy storage device is to: the fault ride-through capability of the microgrid is improved; installing a fault current limiter can help limit the fault current caused by the microgrid and simultaneously compensate for voltage dips at the point of common coupling; when the microgrid is still in a networking mode, the energy storage system acts on stabilizing power fluctuation in the microgrid, namely an active-reactive power control mode is adopted; wherein, adopt two closed loop power controller in order to produce the reference value of current signal to realize the quick tracking of active power, reactive power, mathematical equation can be expressed as:

<math> <mrow> <msubsup> <mi>i</mi> <mi>d</mi> <mo>*</mo> </msubsup> <mo>=</mo> <msub> <mi>K</mi> <mrow> <mi>p</mi> <mi>v</mi> </mrow> </msub> <mrow> <mo>(</mo> <msubsup> <mi>V</mi> <mrow> <mi>l</mi> <mi>d</mi> </mrow> <mo>*</mo> </msubsup> <mo>-</mo> <msub> <mi>v</mi> <mrow> <mi>l</mi> <mi>d</mi> </mrow> </msub> <mo>)</mo> </mrow> <mo>+</mo> <msub> <mi>K</mi> <mrow> <mi>i</mi> <mi>v</mi> </mrow> </msub> <mo>&Integral;</mo> <mrow> <mo>(</mo> <msubsup> <mi>V</mi> <mrow> <mi>d</mi> <mi>l</mi> </mrow> <mo>*</mo> </msubsup> <mo>-</mo> <msub> <mi>v</mi> <mrow> <mi>l</mi> <mi>d</mi> </mrow> </msub> <mo>)</mo> </mrow> <mi>d</mi> <mi>t</mi> </mrow> </math> formula II

<math> <mrow> <msubsup> <mi>i</mi> <mi>q</mi> <mo>*</mo> </msubsup> <mo>=</mo> <msub> <mi>K</mi> <mrow> <mi>p</mi> <mi>v</mi> </mrow> </msub> <mrow> <mo>(</mo> <msubsup> <mi>V</mi> <mrow> <mi>l</mi> <mi>q</mi> </mrow> <mo>*</mo> </msubsup> <mo>-</mo> <msub> <mi>v</mi> <mrow> <mi>l</mi> <mi>q</mi> </mrow> </msub> <mo>)</mo> </mrow> <mo>+</mo> <msub> <mi>K</mi> <mrow> <mi>i</mi> <mi>v</mi> </mrow> </msub> <mo>&Integral;</mo> <mrow> <mo>(</mo> <msubsup> <mi>V</mi> <mrow> <mi>l</mi> <mi>q</mi> </mrow> <mo>*</mo> </msubsup> <mo>-</mo> <msub> <mi>v</mi> <mrow> <mi>l</mi> <mi>q</mi> </mrow> </msub> <mo>)</mo> </mrow> <mi>d</mi> <mi>t</mi> </mrow> </math> Formula III

Wherein, K_PVAnd K_ivIs the proportional and integral coefficients of the voltage controller; v^* _ldAnd V^* _lqD-axis and q-axis reference signals representing a direct current voltage in the direct current power supply processing device;

selecting step 2: when a short-circuit fault occurs at the upstream of a connecting line at a public coupling point or a power distribution system connected with a microgrid, a relay protection device configured on the connecting line trips off a static switch after several power frequency cycles, and the microgrid needs to be separated from a main network; the current limiter transmits an action signal to the rapid energy storage device while inhibiting the rise of fault current and improving the voltage drop, so that the energy storage device is switched to a droop control mode to stabilize the frequency and the voltage of the microgrid and realize the smooth transition of the microgrid between a grid-connected mode and an island mode;

after the current limiter acts rapidly, the short-circuit current can be reasonably inhibited, and meanwhile, the voltage drop of the public coupling point can be effectively improved, so that the method has positive significance for stably transitioning the micro-grid to the isolated grid state; then, the static switch action enables the micro-grid to be off-grid, and the energy storage system is switched from active and reactive control during grid connection to droop control, so that the voltage and the frequency of the micro-grid are stabilized; reference voltage E^*And a reference frequency omega of the point of common coupling^*Can be obtained by the following formula:

ω^*＝ω₀-K_ωp type IV

E^*＝E₀-K_EQ type five

Wherein, K_ωAnd K_EProportional coefficients of the frequency controller and the voltage controller are respectively; p and Q represent the active and reactive power of the fast energy storage system.