CN113437761A - Island microgrid transient stability improving method considering mode switching - Google Patents

Island microgrid transient stability improving method considering mode switching Download PDF

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CN113437761A
CN113437761A CN202110819396.2A CN202110819396A CN113437761A CN 113437761 A CN113437761 A CN 113437761A CN 202110819396 A CN202110819396 A CN 202110819396A CN 113437761 A CN113437761 A CN 113437761A
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inverter
grid
virtual
inertia
island
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CN113437761B (en
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赵峰
帅智康
赵慧敏
彭也伦
程慧婕
沈阳
沈超
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Hunan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/002—Flicker reduction, e.g. compensation of flicker introduced by non-linear load
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]

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  • Power Engineering (AREA)
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  • Nonlinear Science (AREA)
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Abstract

The invention discloses an island microgrid transient stability improving method considering mode switching, which comprises the following steps of 1: acquiring a mathematical model between virtual inertia of a virtual synchronous motor and total energy of an island micro-grid system; 2: monitoring the voltage of a grid-connected port point of an inverter in real time, and judging whether the island microgrid system has a fault; 3: when a fault is judged to occur, the inverter is switched to hysteresis control, and the virtual inertia of the virtual synchronous motor is adjusted to improve the transient stability of the inverter; when the voltage amplitude of the grid-connected port point of the inverter is monitored to deviate from the rated voltage by less than +/-5 percent, the inverter is switched from hysteresis control to virtual synchronous control, and inertia is dynamically adjusted in the switching process to improve the transient stability of the inverter; when the inverter is judged to normally operate, the inverter is in a virtual synchronous control mode, and the virtual inertia of a virtual synchronous motor is dynamically adjusted to improve the transient stability of the inverter; 4: return to 2 to continue execution. The method is simple to control and has good effect.

Description

Island microgrid transient stability improving method considering mode switching
Technical Field
The invention belongs to the technical field of micro-grids, and relates to an island micro-grid transient stability improving method considering mode switching.
Background
The micro-grid is an important means for solving the energy crisis, the island micro-grid is an energy system consisting of various distributed micro-sources, has the advantages of various energy sources, high energy conversion efficiency, low pollution and the like, and is widely applied to remote areas such as ocean reefs, plateau mountain areas and the like. An inverter is a core unit of an island microgrid, and Virtual Synchronous Generator (VSG) control is an inverter control technology which is rapidly developed and has gained wide attention in recent years.
The virtual synchronous inverter faces short-circuit fault impact in the operation process, and the generated fault current impact is large and the rising speed is high. Since the core unit of the inverter is a power electronic device, the inverter is weak in overcurrent capability and is easily burnt during a transient state. Therefore, to avoid burning out the inverter with a large inrush current, a control strategy based on mode switching is proposed. The basic idea of this mode switching strategy is: and switching the VSG control to the hysteresis current control when the fault occurs, and switching the control strategy of the inverter from the hysteresis current control to the VSG control after the fault is ended. The control strategy of using the mode switching improves the fault impact resistance of the inverter, but also brings transient stability problems. At present, transient stability operation characteristics of an island micro-grid system considering mode switching are not researched, and an island micro-grid transient stability improvement method considering mode switching is also needed to be proposed.
Typical inverter current limiting strategies are: virtual impedance control, current limiters, mode switching, etc. [ CN107437821A, CN106655272A ]. Using different current limiting strategies has different effects on the inverter system. Research on the transient stability analysis and stability improvement method of the inverter or the inverter system by considering the virtual impedance and the current limiter is carried out [ CN105826949A, CN105356781], but the transient stability research of the microgrid system by considering mode switching is not reported yet. In the prior research, a control optimization method considering multi-state switching of an alternating current-direct current hybrid power grid [ CN107579519B ], but research on system stability considering state switching control and a stability improvement method thereof aiming at an island alternating current micro-grid is not reported.
Disclosure of Invention
The invention provides an isolated island microgrid transient stability improving method considering mode switching, an inverter usually adopts a mode switching control strategy to limit the current of the inverter, and the current limiting research of a converter is mostly concentrated on the current limiting level of a converter, but the stability of a system cannot be considered and improved while the current is limited, so that the isolated island microgrid adopting the existing mode switching control strategy has the problem of low stability. On the basis of a traditional current limiting method based on mode switching control, the stability problem of a converter is fully considered, and an island micro-grid transient stability improving method considering mode switching is provided. The problem of low stability when current mode switching control technique is applied to island microgrid is solved.
The invention adopts the technical scheme that an island microgrid transient stability improving method considering mode switching comprises the following steps:
step 1: acquiring a mathematical model of a relation between virtual inertia of a virtual synchronous motor in an island micro-grid system and total energy of the island micro-grid system;
step 2: monitoring the voltage of a grid-connected port point of an inverter in an island micro-grid system in real time, and judging whether the island micro-grid system fails;
when the island micro-grid system is judged to be in fault in the step 2, the inverter is switched to a hysteresis control mode, and the virtual inertia of the virtual synchronous motor is dynamically adjusted to improve the transient stability of the inverter; when the voltage amplitude of the grid-connected port point of the inverter is monitored to deviate from the rated voltage by less than +/-5 percent, the inverter is switched into a virtual synchronous control mode from hysteresis control, and the transient stability of the inverter is improved by dynamically adjusting inertia in the switching process;
when the islanding micro-grid system is judged to normally operate in the step 2, the inverter is in a virtual synchronous control mode, and the virtual inertia of the virtual synchronous motor is dynamically adjusted to improve the transient stability of the inverter;
and 4, step 4: and returning to the step 2 to continue the execution.
Further, the mathematical model for acquiring the relationship between the virtual inertia of the virtual synchronous motor in the islanding micro-grid system and the total energy of the islanding micro-grid system in the step 1 is specifically as follows:
when the island micro-grid system does not break down, the switch S of the island micro-grid is controlled2And switch S2-1Each is connected to a contact 1 of the islanded microgrid to control a switch S of the islanded microgridMThe inverter is connected to a contact 1 of the inverter, the inverter operates in a virtual synchronous control mode, and a swing equation of an equivalent single-machine infinite system of the island micro-grid is as follows:
Figure BDA0003171313540000021
wherein t is time; meqIs an equivalent virtual inertia of a virtual synchronous motor, and
Figure BDA0003171313540000022
wherein M isSGIs the inertia of the synchronous generator; mVSGIs the virtual inertia of the virtual synchronous motor;
PMis equivalent reference active power of equivalent single-machine infinite system, and
Figure BDA0003171313540000031
wherein, PMSGIs the reference active power of the synchronous generator; pMVSGIs the reference active power of the virtual synchronous motor;
PINVis the inherent power difference between the synchronous generator and the virtual synchronous motor due to the characteristic difference thereof, and
Figure BDA0003171313540000032
wherein E is1An equivalent internal potential that is an equivalent internal potential point of the synchronous generator; e2An equivalent internal potential at an equivalent internal potential point for the virtual synchronous machine; g11The self-conductance being the equivalent internal potential point of the synchronous generator; g22Is the self-conductance of the virtual synchronous motor at the equivalent internal potential point;
Pemis the maximum output power of equivalent single-machine infinite system, and
Figure BDA0003171313540000033
wherein phi is12The phase angle difference between the equivalent internal potential point of the synchronous generator and the equivalent internal potential point of the virtual synchronous motor is obtained;
gamma is a virtual work angle difference, and
Figure BDA0003171313540000034
δ is the phase angle difference between the virtual synchronous machine and the synchronous generator, i.e. the power angle:
δ＝δ1-δ2
wherein, delta1Is the phase angle of the synchronous generator; delta2Is the phase angle of the virtual synchronous motor;
when the island micro-grid is faultless, the equivalent reference active power is as follows:
Figure BDA0003171313540000035
let the equivalent power angle δ':
Figure BDA0003171313540000036
equation (1) can be simplified to:
Figure BDA0003171313540000037
wherein, PemgThe output power of an equivalent single-machine infinite system;
the total energy of the unstable balance point when the island microgrid has no fault is as follows:
Figure BDA0003171313540000041
wherein, deltasThe power angle of a stable balance point S of an equivalent single machine infinite system of the island microgrid;
when the islanding micro-grid system has serious faults, the switch S of the islanding micro-grid is controlled2And switch S2-1Each is connected to its own contact 2 to control the switch S of the island micro-gridMWhen the isolated island microgrid is connected to a contact 2 of the isolated island microgrid, the inverter is switched from a virtual control mode to a hysteresis control mode, and the total energy of the isolated island microgrid at a stable balance point of the hysteresis control mode is as follows:
Figure BDA0003171313540000042
wherein, delta'C1An equivalent work angle, δ ', of a stable equilibrium point of the modulator in the hysteresis control mode'S1The equivalent power angle is a stable balance point when the island micro-grid has no fault;
Vk|C1for the island micro-grid to stabilize the kinetic energy of the balance point in the hysteresis control mode, and
Figure BDA0003171313540000043
wherein, Vk|S1The kinetic energy of a stable balance point when the island micro-grid has no fault;
Figure BDA0003171313540000044
equivalent power angular acceleration of an equivalent single machine infinite system of an island micro-grid;
Vp|C1for the island micro-grid, the potential energy of the balance point is stabilized in a hysteresis control mode, and
Figure BDA0003171313540000045
the total boundary energy V of the islanded microgrid systemsumComprises the following steps:
Figure BDA0003171313540000046
further simplifying to obtain a mathematical model between the virtual inertia of the virtual synchronous motor and the total boundary energy of the islanding micro-grid system, wherein the mathematical model comprises the following steps:
Figure BDA0003171313540000047
further, the total boundary energy of the island microgrid system is gradually reduced along with the increase of the virtual inertia of the virtual synchronous motor.
Further, the step 2 specifically comprises:
when the voltage amplitude of the grid-connected port point of the inverter deviates from the rated voltage by more than +/-5 percent, judging that the island micro-grid system has a fault;
and when the voltage amplitude of the grid-connected port point of the inverter deviates from the rated voltage by less than +/-5 percent, judging that the islanding micro-grid system normally operates.
Further, when the islanding micro-grid system is judged to have a fault, the virtual inertia of the virtual synchronous motor is set as the transient inertia of the inverter, so that the output frequency of the equivalent single-machine infinite system swing amplitude equation of the islanding micro-grid can quickly track the bus voltage, and the mathematical model is as follows:
MVSG＝Mminhysteresis control mode (7)
Wherein M isminIs the transient inertia of the inverter.
Further, when the inverter is switched from the hysteresis control to the virtual synchronous control mode: if the output active power of the inverter changes too fast, the inverter loses stability, and the dynamic change rate of the actual output power of the inverter
Figure BDA0003171313540000051
Is still greater than the maximum allowable change rate P of the output active power of the inverterjThe virtual inertia of the virtual synchronous motor is set as the transient inertia M of the inverterminSo that the inverter can be quickly recovered and stabilized; dynamic rate of change of actual power output by inverter
Figure BDA0003171313540000052
Equal to or less than the maximum allowable rate of change P of the active power output by the inverterjThe virtual inertia of the virtual synchronous motor is set to be the rated reference inertia M of the virtual synchronous motorrvsgWherein P isEINVIs the actual power output by the inverter.
Further, when the islanding micro-grid system normally operates, the inverter operates in a virtual synchronous control mode, and the inverter outputs the actual power dynamic change rate
Figure BDA0003171313540000053
Maximum allowable change rate P greater than inverter output active powerjThe virtual inertia of the virtual synchronous motor is set to be the rated reference inertia M of the virtual synchronous motorrvsg(ii) a When the inverter operates in the virtual synchronous control mode and the inverter outputs the dynamic change rate of the actual power
Figure BDA0003171313540000054
Maximum allowable change rate P of active power output by inverterjThe virtual inertia of the virtual synchronous motor is set as the transient inertia M of the invertermin(ii) a The mathematical model is as follows:
Figure BDA0003171313540000061
the invention has the beneficial effects that: the embodiment of the invention researches an isolated island micro-grid, researches the influence of mode switching on the transient stability of an isolated island micro-grid system, and provides an isolated island micro-grid transient stability improving method considering mode switching control. The adjusting process of the invention depends on automatic control of a program, namely, the inertia is adjusted in a self-adaptive manner according to the change speed of the output power of the inverter, and the transient stability of the island microgrid can be effectively improved without extra hardware. Compared with the traditional method, the method is simple to control, does not need additional hardware and has a good control effect.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic diagram of an islanded microgrid system;
fig. 2 is an equivalent circuit diagram of an SG-VSG parallel system in consideration of mode switching control;
fig. 3 is a schematic diagram of an island microgrid flexible inertia control method provided by an embodiment of the invention;
FIG. 4 is a power angle curve P of an equivalent single machine infinite systememg- δ' isoarea plot;
FIG. 5 is a virtual inertia M of the inverterVSGAnd boundary energy VsumA schematic diagram of the relationship of (1);
FIG. 6 is a schematic diagram of the switching between the virtual synchronous control mode and the hysteresis current control mode of the inverter;
FIG. 7 is a schematic of inverter a-phase output voltage and current using a conventional mode switching strategy;
FIG. 8 is a schematic of the output active power versus reactive power of an inverter using a conventional mode switching strategy;
fig. 9 is a schematic diagram of an island microgrid flexible inertia control method provided by an embodiment of the invention in improving a-phase output voltage and current.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Fig. 1 is a schematic diagram of an island microgrid system, which is composed of a Synchronous Generator (SG), a Virtual Synchronous Generator (VSG) and a common Load (Load), wherein a node (i) is an equivalent internal potential point of the Synchronous Generator (SG), a node (ii) is an equivalent internal potential point of the Virtual Synchronous Generator (VSG), and a node (iii) is a common connection point. X13Is an equivalent line inductance, R, between the node (i) and the node (iii)13Is the equivalent line resistance, X, between the node (i) and the node (iii)23Is the equivalent line inductance between the node (R) and the node (R)23Is equivalent line resistance between a node II and a node III, j is a virtual axis vector unit, a synchronous motor (SG) is accessed to the node I, a virtual synchronous motor (VSG) is accessed to the node II, a common Load (Load) is accessed to the node III, the node I and the node IIAnd a power transmission line exists between the third point and the third point, a power transmission route also exists between the third point and the third point, and the synchronous motor (SG) and the virtual synchronous motor (VSG) are connected in parallel to jointly supply power to a common Load (Load).
The fault current increases rapidly, the power semiconductor overcurrent capacity is small, and the inverter may be burned. Therefore, mode switching control is employed to prevent the inverter from overcurrent. As shown in fig. 2, the SG-VSG parallel island microgrid equivalent circuit diagram considering mode switching control considers that a Synchronous Generator (SG) and a virtual synchronous motor (VSG) are equivalent voltage sources when the SG-VSG island microgrid operates in a normal mode. Wherein the content of the first and second substances,
Figure BDA0003171313540000072
is the equivalent internal potential at the Synchronous Generator (SG) node (r); y is13Is the equivalent admittance between the first node and the third node; y is32Is the equivalent admittance between the node II and the node III; y is30The equivalent admittance between the node (c) and the public ground when the virtual synchronous motor (VSG) is in the normal working mode;
Figure BDA0003171313540000073
is the equivalent internal potential of the virtual synchronous machine (VSG) at node two; when a serious three-phase symmetrical grounding short circuit grounding fault occurs at the node III, the output current of the Synchronous Generator (SG) and the virtual synchronous motor (VSG) is obviously increased, the control mode of the inverter is switched from the virtual synchronous control mode to the hysteresis control mode, and the virtual synchronous motor (VSG) is an equivalent current source at the moment, wherein y is30-1Is the equivalent admittance between a node (c) and the public ground when a virtual synchronous motor (VSG) is in a hysteresis current control mode;
Figure BDA0003171313540000071
the inverter is used for equivalently outputting current when a virtual synchronous motor (VSG) is in a hysteresis current control mode. When the ground fault is cleared, the virtual synchronous motor (VSG) is switched from the hysteresis control mode back to the virtual synchronous control mode, i.e., back to the normal operation state.
FIG. 3 shows an isolated island micro-power system with mode switching taken into accountSchematic diagram of network transient stability improvement method, wherein PMVSGA reference active power for virtual synchronous machine (VSG) control; psetTo track active power; mVSGIs a virtual inertia of a virtual synchronous motor (VSG); mrvsgIs a nominal reference inertia of a virtual synchronous motor (VSG); mminIs the transient inertia of the inverter, and Mmin＜Mrvsg；PjOutputting a maximum allowable rate of change of active power for the inverter; moutThe inertia is output inertia of the variable inertia control module; thetanRated angular frequency for the inverter; t ismIs an equivalent reference torque; t iseIs an electromagnetic torque; dPIs a damping coefficient; s is a laplace operator; theta is the inverter phase; pEINVActual power output for the inverter; omegagThe frequency is the grid-connected point frequency of the VSG inverter; s1、S2And SMAre all state change-over switches. The method comprises the following steps:
step 1: acquiring a mathematical model of a relation between virtual inertia of a virtual synchronous motor and total energy of an island micro-grid system;
when the isolated island micro-grid system is not in fault, the switch S2Switch S2-1Each being connected to its own contact 1, switch SMWhen the isolated island micro-grid is connected to a contact 1 of the isolated island micro-grid, the inverter operates in a virtual synchronous control mode, and a swing equation of an equivalent single-machine infinite system of the isolated island micro-grid is as follows:
Figure BDA0003171313540000081
wherein t is time; meqIs an equivalent virtual inertia of a virtual synchronous motor (VSG), and
Figure BDA0003171313540000082
wherein M isSGInertia of a Synchronous Generator (SG); mVSGIs a virtual inertia of a virtual synchronous motor (VSG);
PMis equivalent reference active power of an equivalent single-machine infinite system, and
Figure BDA0003171313540000083
wherein, PMSGIs a reference active power of a Synchronous Generator (SG); pMVSGA reference active power for a virtual synchronous machine (VSG);
PINVis the inherent power difference between the Synchronous Generator (SG) and the Virtual Synchronous Generator (VSG) due to the characteristic difference thereof, and
Figure BDA0003171313540000084
wherein E is1Is the equivalent internal potential of the Synchronous Generator (SG) at node (r); e2Equivalent internal potential at node two for a virtual synchronous machine (VSG); g11Is the self-conductance of the node I; g22Is the self-conductance of node two;
Pemis the maximum output power of equivalent single-machine infinite system, and
Figure BDA0003171313540000091
wherein phi is12Is the phase angle difference between the first node and the second node; the phase angle difference between the node phi and the node phi21And phi is12＝φ21；
Gamma is a virtual work angle difference, and
Figure BDA0003171313540000092
δ is the phase angle difference between the virtual synchronous machine (VSG) and the Synchronous Generator (SG), i.e. the power angle:
δ＝δ1-δ2 (7)
wherein, delta1Is the phase angle of the Synchronous Generator (SG); delta2Is the phase angle of a virtual synchronous motor (VSG);
when the island micro-grid is faultless, the equivalent reference active power is as follows:
Figure BDA0003171313540000097
let the equivalent power angle δ':
Figure BDA0003171313540000093
equation (1) can be simplified to:
Figure BDA0003171313540000094
wherein, PemgThe output power of an equivalent single-machine infinite system;
equation (10) is equivalent to the power angle curve P of a single-machine infinite systememgThe equal area of-delta' is shown in fig. 4, where,
Figure BDA0003171313540000095
equivalent reference active power is obtained when the island micro-grid has no fault; s1The isolated island micro-grid fault-free stable balance point is a stable balance point when the isolated island micro-grid has no fault; u shape1The method is an unstable balance point when the island micro-grid has no fault;
Figure BDA0003171313540000096
equivalent reference active power is provided for an island microgrid in an inverter hysteresis control mode; c1Is a stable balance point of the inverter in the hysteresis control mode; t is a stable balance point C of the slave inverter in the hysteresis control mode1Stable balance point S when operating to island micro-grid without fault1The equivalent acceleration area of (d); w is a stable balance point S when the isolated island micro-grid has no fault1Unstable balance point U when operating to island micro-grid without fault1Equivalent of (2)A deceleration area; delta 'of'c1Is PemgStable equilibrium point C of the inverter in hysteresis control mode on the delta' curve1Of equivalent power angle of δ's1Is PemgStable balance point S for island micro-grid without fault on delta curve1Of equivalent power angle of δ'u1Is PemgUnstable balance point U of islanding microgrid on delta' curve when no fault exists1The equivalent power angle of (c). When the system is not in fault, the unstable balance point U is generated when the isolated island micro-grid is not in fault1The total energy of (a) is the area denoted by T in fig. 4, and is:
Figure BDA0003171313540000101
wherein, deltasThe power angle of a stable balance point S of an equivalent single-machine infinite system of an island micro-grid is disclosed.
When severe fault occurs to the islanding micro-grid system, the switch S2And switch S2-1Each being connected to its own contact 2, switch SMSwitched on to its own contact 2, the inverter switches from the virtual control mode to the hysteresis control mode. When the inverter is switched to the hysteresis control, the process is discontinuous. Using mode switching control to implement the ride-through process is equivalent to a process of changing the inverter reference active power. Therefore, if the influence of the voltage control loops of the Synchronous Generator (SG) and the virtual synchronous machine (VSG) on the transient stability is ignored, the operating point of the island microgrid after the fault based on the SG-VSG parallel connection is also at PemgOn the delta' curve, the equivalent is the system reference active power of
Figure BDA0003171313540000102
When the control mode of the inverter is switched from the virtual synchronous control mode to the hysteresis control mode, the working point of the island microgrid is switched to a stable balance point C when the hysteresis control of the inverter is carried out1. And after the fault is cleared, the control mode of the inverter is switched from the hysteresis control mode to the virtual synchronous control mode. As can be seen from FIG. 4, when the inverter control strategy is restored to the virtual synchronous control mode from the hysteresis controlIn time, the reference active power of the inverter also changes, so that the operation point of the island microgrid is from a stable balance point C in the hysteresis control mode1Stable balance point S when changing into SG-VSG parallel system without fault1. Stable balance point C of island microgrid in hysteresis control mode based on SG-VSG parallel connection1When the inverter operates, the inverter adopts a hysteresis control mode, so that the inverter outputs current phase tracking bus voltage. Therefore, the island microgrid stabilizes the balance point C in the hysteresis control mode1The kinetic energy of (a) is:
Figure BDA0003171313540000103
wherein, Vk|S1The kinetic energy of a stable balance point when the island micro-grid has no fault;
Figure BDA0003171313540000104
equivalent power angular acceleration of an equivalent single-machine infinite system of an island micro-grid;
stable balance point C of island micro-grid in hysteresis control mode1The potential energy of (A) is as follows:
Figure BDA0003171313540000105
from equations (12) and (13), the stable equilibrium point C of the inverter using the hysteresis control mode is known1The total energy of (a) is the area denoted by W in fig. 4, and is:
Figure BDA0003171313540000111
total boundary energy V of island micro-grid systemsumComprises the following steps:
Figure BDA0003171313540000112
the criterion of the stability of the islanding microgrid system obtained by the formula (15) is as follows:
case 1: when V issumIf the voltage is more than 0, the parallel system returns to a stable balance point S when the isolated island micro-grid has no fault1；
Case 2: when V issumIf the angle is less than 0, the parallel system loses synchronization and the angle is unstable.
Case 3: when V issumThe parallel system operates at a critical steady state, 0.
The bus voltage amplitude is set to be a fixed value. The virtual inertia M of the virtual synchronous motor (VSG) can be obtained from the equations (11), (14) and (15)VSGTotal boundary energy V with island microgrid systemsumThe mathematical model in between is:
Figure BDA0003171313540000113
v expressed by equation (16) when the rated capacity of the virtual synchronous machine (VSG) is increased from 0 to 150kWsumAnd MVSGThe graph of the relationship is shown in FIG. 5, in which the horizontal axis represents the virtual inertia M of the virtual synchronous motor (VSG)VSGAnd the vertical axis is the total boundary energy V of the island micro-grid systemsum。VsumIs an important index for evaluating the stability of the SG-VSG parallel system when V issumWhen the maximum deceleration area is larger than 0, the maximum deceleration area of the system is larger than the acceleration area, and the system is kept stable. When V issumWhen the maximum deceleration area of the system is smaller than 0, the maximum deceleration area of the system is smaller than the acceleration area, and the system is unstable. Thus, VsumThe larger the stability margin representing the system, and thus the more severe transient impacts the system can withstand, the higher the transient stability of the system. As can be seen from FIG. 5, following MVSGIncrease of VsumThe gradual decrease indicates that the transient stability of the parallel system decreases with the increase of the virtual inertia. As shown in fig. 5, when the inertia of the virtual synchronous motor (VSG) is gradually increased and P is made to be increasedem＜P′MMor-Pem＞P′M-c1In time, there is a power transmission limit region and the system is necessarily unstable. It can thus be derived: the use of a smaller virtual inertia for the Virtual Synchronous Generator (VSG) helps to improve the transient stability of the inverter.
Step 2: monitoring the voltage of a grid-connected port point of an inverter in real time, and judging whether the island microgrid system has a fault;
when the voltage amplitude of the grid-connected port point of the inverter deviates from the rated voltage by more than +/-5 percent, judging that the island micro-grid system has a fault;
and when the voltage amplitude of the grid-connected port point of the inverter deviates from the rated voltage by less than +/-5 percent, judging that the islanding micro-grid system normally operates.
And step 3: when the islanding micro grid system is judged to have a fault in the step 2, the inverter is switched to a hysteresis control mode, and the virtual inertia M of the virtual synchronous motor (VSG) is dynamically adjustedVSGThe transient stability of the inverter is improved;
when the island micro-grid system fails, the active power output by the inverter fluctuates greatly, and the switch S2And S2-1At its own contact "2", switch SMAt the contact point '2' of the inverter, the inverter is in a hysteresis control mode, and the virtual inertia M of a virtual synchronous motor (VSG)VSGTransient inertia M set as inverterminThe output frequency of an equivalent single-machine infinite system swing amplitude equation of an island micro-grid is enabled to quickly track the bus voltage, and the mathematical model is as follows:
MVSG＝Mminhysteresis control mode (17)
When the voltage amplitude of a grid-connected port point of the inverter is monitored to deviate from the rated voltage by less than +/-5 percent, the inverter is switched to a virtual synchronous control mode from hysteresis control, inertia is dynamically adjusted, and the transient stability of the inverter is improved, specifically:
when the voltage amplitude of the grid-connected port point of the inverter is detected to deviate from the rated voltage amplitude by less than +/-5 percent, the switch S2And S1Switched from the position of '2' to '1', the switch SMSwitching from the position of '2' to the position of '1', switching the inverter from the hysteresis control mode to the virtual synchronous control mode, and switching the virtual synchronous control mode and the hysteresis control of the inverterThe schematic diagram of the mode switching is shown in fig. 6. Wherein e isa，b，cThe neutral point potential of an a/b/c three-phase bridge arm of the inverter is shown; vga，b，cThe voltage is the a/b/c three-phase voltage of the grid-connected point of the inverter; i.e. isa，b，cTracking a virtual current of an inverter hysteresis control current for the inverter; rsIs a virtual resistance; l issIs a virtual inductor; i.e. isetControlling a current reference for hysteresis; i.e. iouta，b，cIs the output current of the inverter; phi is ahypAn offset angle of a hysteresis control mode; vgIs the inverter grid-connected point voltage.
The phase of the hysteresis current is determined by the phase locked loop. Due to PsetThe tracking active power is obtained by tracking the active power actually output by the inverter through the tracking module, so that P is obtained when the fault is clearedset＝PEINV. By making P during hysteresis control modeset＝PEINVSo that the phases of the inverter phase theta and the grid-connected point voltage maintain a relatively fixed phase angle difference. Seamless switching of the inverter from the hysteresis current mode to the virtual synchronous control mode can be realized.
When the control mode of the inverter is switched from the hysteresis control mode to the virtual synchronous control mode, if the output active power of the inverter changes too fast, the inverter loses stability, and the dynamic change rate of the actual power output by the inverter
Figure BDA0003171313540000121
Is still greater than the maximum allowable change rate P of the output active power of the inverterjVirtual inertia M of virtual synchronous motor (VSG)VSGTransient inertia M set as inverterminSo as to quickly restore the inverter to be stable and wait for the inverter to output the dynamic change rate of the actual power
Figure BDA0003171313540000122
Equal to or less than the maximum allowable rate of change P of the active power output by the inverterjVirtual inertia M of virtual synchronous motor (VSG)VSGRated reference inertia M set as virtual synchronous motor (VSG)rvsg. At this time, the operation state and failure of the inverterThe fault operation states of the front inverters are completely consistent.
When the islanding micro-grid system is judged to normally operate in step S2, the inverter is in a virtual synchronous control mode, and the virtual inertia M of the virtual synchronous motor (VSG) is dynamically adjustedVSGThe transient stability of the inverter is improved; the method specifically comprises the following steps:
monitoring the dynamic change rate of the actual power output by the inverter to realize dynamic adjustment of the inertia constant, and when the SG-VSG parallel system operates in a virtual synchronous control mode, the dynamic change rate of the actual power output by the inverter
Figure BDA0003171313540000131
Maximum allowable change rate P greater than inverter output active powerjVirtual inertia M of virtual synchronous motor (VSG)VSGRated reference inertia M set as virtual synchronous motor (VSG)rvsg(ii) a When the SG-VSG parallel system operates in a virtual synchronous control mode, and the inverter outputs the actual power dynamic change rate
Figure BDA0003171313540000132
Maximum allowable change rate P of active power output by inverterjVirtual inertia M of virtual synchronous motor (VSG)VSGTransient inertia M set as invertermin(ii) a The mathematical model is as follows:
Figure BDA0003171313540000133
step S4: return to step S2 to continue execution.
FIG. 7 shows the output voltage V of phase a of an inverter using a conventional mode switching strategycaAnd current ifaSchematic representation. The parameters of the synchronous machine (SG) and the virtual synchronous machine (VSG) in the test are shown in table 1, and the short-circuit impedance is equal to 0.1 Ω to simulate a ground short fault. The duration of the ground fault is 0.5 s. When a ground fault occurs and then the bus voltage drops, the control mode of the inverter will be switched from the virtual synchronous control mode to the hysteresis control mode. To support the bus voltageThe output reactive power of the inverter is required, and the offset angle of the hysteresis control is set to pi/4. When the control mode of the inverter is switched from hysteresis control to virtual synchronous control mode, the output active power P of the inverterEINVAnd reactive power QEINVWill oscillate periodically as shown in fig. 8. The inverter using the variable inertia control method proposed in this embodiment can improve the transient power angle stability of the inverter, and at this time, the waveform of the a-phase output voltage and the output current of the inverter is as shown in fig. 9. When the system is in fault, the rotational inertia of the swing equation of the inverter is set to be Mmin. After clearing the fault, the bus voltage will rise and the control mode of the inverter switches from the hysteresis control mode to the virtual synchronous control mode. This shows that after the control mode of the inverter is switched back to the virtual synchronous control mode, the inverter dynamically adjusts the virtual inertia according to the change rate of the output power of the inverter, thereby improving the transient stability of the inverter.
As can be seen from a comparison between fig. 7 and fig. 9, fig. 7 shows waveforms of the conventional virtual synchronous inverter without using the variable inertia control method during fault ride-through, and output voltage and current continuously oscillate after the inverter is switched to the virtual synchronous control, which indicates that a power angle instability problem occurs in a system where the inverter is located. Fig. 9 shows waveforms of the virtual synchronous inverter using the variable inertia control method during fault ride-through, and the output voltage and current of the virtual synchronous inverter are switched to the virtual synchronous control mode in the inverter control mode to gradually stabilize and recover to the level before the fault, so that the system does not have the instability problem. Therefore, the effectiveness of the SG-VSG microgrid transient stability improving method provided by the patent is verified. The method effectively avoids the transient instability problem possibly brought by the traditional method, thereby greatly improving the transient stability of the island micro-grid
TABLE 1 parameters of SG and VSG
Parameter(s) Means of Numerical value
PMSG Reference active power of SG 30kW
PMVSG Reference active power of VSG 20kW
QMVSG Reference reactive power of VSG 5kvar
Dp Damping coefficient of VSG 20
Dq Droop coefficient of VSG reactive control loop 166kvar/V
Rs+jLs Virtual impedance (0.05+0.942j)Ω
φhyp Hysteresis controlled deflection angle π/4
Y13 Admittance between nodes (0.067-1.057j)S
Y32 Admittance between nodes 2 and III (0.07-0.265j)S
Y30 Admittance of node to ground (0.365-0.043j)S
Zgnd Short circuit impedance 0.1Ω
MSG Inertial time constant of SG 100s
MVSG Inertia time constant of VSG 10s
The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims (7)

1. An island micro-grid transient stability improving method considering mode switching is characterized by comprising the following steps:
step 1: acquiring a mathematical model of a relation between virtual inertia of a virtual synchronous motor in an island micro-grid system and total energy of the island micro-grid system;
step 2: monitoring the voltage of a grid-connected port point of an inverter in an island micro-grid system in real time, and judging whether the island micro-grid system fails;
when the island micro-grid system is judged to be in fault in the step 2, the inverter is switched to a hysteresis control mode, and the virtual inertia of the virtual synchronous motor is dynamically adjusted to improve the transient stability of the inverter; when the voltage amplitude of the grid-connected port point of the inverter is monitored to deviate from the rated voltage by less than +/-5 percent, the inverter is switched into a virtual synchronous control mode from hysteresis control, and the transient stability of the inverter is improved by dynamically adjusting inertia in the switching process;
when the islanding micro-grid system is judged to normally operate in the step 2, the inverter is in a virtual synchronous control mode, and the virtual inertia of the virtual synchronous motor is dynamically adjusted to improve the transient stability of the inverter;
and 4, step 4: and returning to the step 2 to continue the execution.
2. The method for improving transient stability of an island micro-grid considering mode switching according to claim 1, wherein the mathematical model for obtaining the relation between the virtual inertia of the virtual synchronous motor in the island micro-grid system and the total energy of the island micro-grid system in the step 1 is specifically as follows:
when the island micro-grid system does not break down, the switch S of the island micro-grid is controlled2And switch S2-1Each is connected to a contact 1 of the islanded microgrid to control a switch S of the islanded microgridMThe inverter is connected to a contact 1 of the inverter, the inverter operates in a virtual synchronous control mode, and a swing equation of an equivalent single-machine infinite system of the island micro-grid is as follows:
Figure FDA0003171313530000011
wherein t is time; meqIs an equivalent virtual inertia of a virtual synchronous motor, and
Figure FDA0003171313530000012
wherein M isSGIs the inertia of the synchronous generator; mVSGIs the virtual inertia of the virtual synchronous motor;
PMis equivalent reference active power of equivalent single-machine infinite system, and
Figure FDA0003171313530000013
wherein, PMSGIs the reference active power of the synchronous generator; pMVSGIs the reference active power of the virtual synchronous motor;
PINVis the inherent power difference between the synchronous generator and the virtual synchronous motor due to the characteristic difference thereof, and
Figure FDA0003171313530000021
wherein E is1An equivalent internal potential that is an equivalent internal potential point of the synchronous generator; e2An equivalent internal potential at an equivalent internal potential point for the virtual synchronous machine; g11The self-conductance being the equivalent internal potential point of the synchronous generator; g22Is the self-conductance of the virtual synchronous motor at the equivalent internal potential point;
Pemis the maximum output power of equivalent single-machine infinite system, and
Figure FDA0003171313530000022
wherein phi is12The phase angle difference between the equivalent internal potential point of the synchronous generator and the equivalent internal potential point of the virtual synchronous motor is obtained;
gamma is a virtual work angle difference, and
Figure FDA0003171313530000023
δ is the phase angle difference between the virtual synchronous machine and the synchronous generator, i.e. the power angle:
δ＝δ1-δ2
wherein, delta1Is the phase angle of the synchronous generator; delta2Is the phase angle of the virtual synchronous motor;
when the island micro-grid is faultless, the equivalent reference active power is as follows:
Figure FDA0003171313530000024
let the equivalent power angle δ':
δ'＝δ-γ
equation (1) can be simplified to:
Figure FDA0003171313530000025
wherein, PemgThe output power of an equivalent single-machine infinite system;
the total energy of the unstable balance point when the island microgrid has no fault is as follows:
Figure FDA0003171313530000026
wherein, deltasThe power angle of a stable balance point S of an equivalent single machine infinite system of the island microgrid;
when the islanding micro-grid system has serious faults, the switch S of the islanding micro-grid is controlled2And switch S2-1Each is connected to its own contact 2 to control the switch S of the island micro-gridMWhen the isolated island microgrid is connected to a contact 2 of the isolated island microgrid, the inverter is switched from a virtual control mode to a hysteresis control mode, and the total energy of the isolated island microgrid at a stable balance point of the hysteresis control mode is as follows:
Figure FDA0003171313530000031
wherein, delta'C1An equivalent work angle, δ ', of a stable equilibrium point of the modulator in the hysteresis control mode'S1The equivalent power angle is a stable balance point when the island micro-grid has no fault;
Vk|C1for the island micro-grid to stabilize the kinetic energy of the balance point in the hysteresis control mode, and
Figure FDA0003171313530000032
wherein, Vk|S1The kinetic energy of a stable balance point when the island micro-grid has no fault;
Figure FDA0003171313530000033
equivalent power angular acceleration of an equivalent single machine infinite system of an island micro-grid;
Vp|C1for the island micro-grid, the potential energy of the balance point is stabilized in a hysteresis control mode, and
Figure FDA0003171313530000034
the total boundary energy V of the islanded microgrid systemsumComprises the following steps:
Figure FDA0003171313530000035
further simplifying to obtain a mathematical model between the virtual inertia of the virtual synchronous motor and the total boundary energy of the islanding micro-grid system, wherein the mathematical model comprises the following steps:
Figure FDA0003171313530000036
3. an island micro-grid transient stability improvement method considering mode switching according to claim 2, characterized in that the total boundary energy of the island micro-grid system is gradually reduced with the increase of virtual inertia of the virtual synchronous machine.
4. The method for improving transient stability of an island micro-grid considering mode switching according to claim 1, wherein the step 2 specifically comprises:
when the voltage amplitude of the grid-connected port point of the inverter deviates from the rated voltage by more than +/-5 percent, judging that the island micro-grid system has a fault;
and when the voltage amplitude of the grid-connected port point of the inverter deviates from the rated voltage by less than +/-5 percent, judging that the islanding micro-grid system normally operates.
5. The method for improving the transient stability of the island microgrid with the mode switching taken into consideration according to claim 1, characterized in that when the island microgrid system is judged to have a fault, the virtual inertia of the virtual synchronous motor is set as the transient inertia of the inverter, so that the output frequency of the equivalent single-machine infinite system swing equation of the island microgrid quickly tracks the bus voltage, and the mathematical model is as follows:
MVSG＝Mminhysteresis control mode (7)
Wherein M isminIs the transient inertia of the inverter.
6. The method for improving transient stability of an island micro-grid considering mode switching according to claim 1, wherein when the inverter is switched from hysteresis control to virtual synchronous control mode: if the output active power of the inverter changes too fast, the inverter loses stability, and the dynamic change rate of the actual output power of the inverter
Figure FDA0003171313530000041
Is still greater than the maximum allowable change rate P of the output active power of the inverterjThe virtual inertia of the virtual synchronous motor is set as the transient inertia M of the inverterminSo that the inverter can be quickly recovered and stabilized; dynamic rate of change of actual power output by inverter
Figure FDA0003171313530000042
Equal to or less than the maximum allowable rate of change P of the active power output by the inverterjThe virtual inertia of the virtual synchronous motor is set to be the rated reference inertia M of the virtual synchronous motorrvsgWherein P isEINVIs the actual power output by the inverter.
7. The method for improving transient stability of an island microgrid with mode switching taken into consideration according to claim 1, characterized in that when the island microgrid system is in normal operation, the inverter operates in a virtual synchronous control mode, and the inverter outputs a dynamic rate of change of actual power
Figure FDA0003171313530000043
Maximum allowable change rate P greater than inverter output active powerjThe virtual inertia of the virtual synchronous motor is set to be the rated reference inertia M of the virtual synchronous motorrvsg(ii) a When the inverter operates in the virtual synchronous control mode and the inverter outputs the dynamic change rate of the actual power
Figure FDA0003171313530000044
Maximum allowable change rate P of active power output by inverterjThe virtual inertia of the virtual synchronous motor is set as the transient inertia M of the invertermin(ii) a The mathematical model is as follows:
Figure FDA0003171313530000051
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