CN114421518A - Virtual synchronous motor fault ride-through and safety protection control strategy - Google Patents

Virtual synchronous motor fault ride-through and safety protection control strategy Download PDF

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Publication number
CN114421518A
CN114421518A CN202111525939.6A CN202111525939A CN114421518A CN 114421518 A CN114421518 A CN 114421518A CN 202111525939 A CN202111525939 A CN 202111525939A CN 114421518 A CN114421518 A CN 114421518A
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voltage
island
frequency
fault ride
harmonic
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卢广陵
孙志媛
周柯
窦骞
文立斌
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Electric Power Research Institute of Guangxi Power Grid Co Ltd
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Electric Power Research Institute of Guangxi Power Grid Co Ltd
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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
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • 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
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management

Abstract

The invention discloses a virtual synchronous motor fault ride-through and safety protection control strategy, which relates to the technical field of smoked synchronous motors and is characterized in that voltage, current and frequency data of PCC are collected; calculating harmonic impedance according to the voltage, current and frequency data; judging whether the harmonic impedance meets an island judgment condition, judging that an island phenomenon occurs when the harmonic impedance meets the island judgment condition, and entering island operation on the premise of allowing the island operation; and otherwise, judging whether the grid fault ride-through FRT operates according to voltage and frequency data, when the FRT fails, if an island system formed by the DER of the distributed power supply and the local load meets island operation conditions, the device can enter the island operation on the premise of allowing the island operation, and otherwise, directly splitting the DER. Effectively identifies island and grid fault ride-through, effectively coordinates the IP and FRT control of DER, and realizes safe and stable grid-connected operation of DER.

Description

Virtual synchronous motor fault ride-through and safety protection control strategy
Technical Field
The invention belongs to the technical field of virtual synchronous motors, and particularly relates to a fault ride-through and safety protection control strategy for a virtual synchronous motor.
Background
In recent years, a distributed power generation technology based on renewable energy sources (such as wind energy, solar energy and the like) is one of important means for human beings to cope with energy crisis and environmental pollution, and in order to feed electric quantity generated by new energy sources into a power grid to the maximum extent, some researchers propose a concept of a Virtual Synchronous Generator (VSG), that is, a corresponding control algorithm is introduced into a grid-connected inverter, so that a distributed power source based on the grid-connected inverter simulates or partially simulates the frequency and voltage control characteristics of the VSG from external characteristics, and meanwhile, grid-connected damping and inertia of the distributed energy sources are enhanced, so that the stability of a distributed system is improved.
For a distributed inverter system, whether the distributed inverter system is out of operation or continues to be in grid-connected operation under a fault condition, the occurrence of an island needs to be accurately and quickly detected. At present, a plurality of island detection methods are suitable for a single inverter of a distributed inverter system, and the island detection methods mainly comprise two types of measurement based on local information and remote communication. The island detection method based on the local information can be divided into a passive type and an active type. The permeability of photovoltaic, wind power and other distributed power sources in a power distribution network is higher and higher, and a scene that a plurality of inverters are operated in parallel appears. Different from the operation of a single inverter, a certain mutual coupling relationship exists among multiple inverters, which affects the detection effect of the island detection method applied to the single inverter, so that a virtual synchronous motor fault ride-through and safety protection control strategy is required.
Disclosure of Invention
The invention aims to provide a virtual synchronous motor fault ride-through and safety protection control strategy, so that the defect that the coordinated operation of the existing virtual synchronous motor island protection and fault ride-through is unstable is overcome.
In order to achieve the above object, the present invention provides a virtual synchronous machine fault ride-through and safety protection control strategy, which comprises the following steps:
s1, collecting voltage, current and frequency data of the PCC;
s2, calculating harmonic impedance according to the voltage, current and frequency data;
s3, judging whether the harmonic impedance meets an island judgment condition, judging that an island phenomenon occurs when the harmonic impedance meets the island judgment condition, and entering island operation on the premise of allowing the island operation; otherwise, the phenomenon of non-island is adopted;
and S4, when a non-isolated island phenomenon occurs, judging whether the grid fault ride-through FRT operates according to the voltage and frequency data, when the FRT fails, if an isolated island system formed by the DER of the distributed power supply and the local load meets an isolated island operation condition, the device can enter the isolated island operation on the premise of allowing the isolated island operation, otherwise, the DER is directly disconnected.
Preferably, the islanding determination condition includes: the harmonic impedance is larger than the set impedance threshold, and the duration is larger than the maximum time limit allowed by normal operation of voltage and frequency.
Preferably, whether the power grid fault ride-through FRT is operated or not is judged according to the voltage and frequency data, and the method specifically includes:
judging whether the voltage and the frequency are in a normal operation range, returning to the initial step for controlling again when the voltage and the frequency belong to the normal operation range, and judging whether to perform power grid fault ride-through when the voltage and the frequency do not belong to the normal operation range;
and when the voltage and the frequency reach the maximum time limit allowed by abnormal operation of the voltage and the frequency, if the voltage and the frequency still do not recover the normal operation range, the fault ride-through of the power grid fails, and otherwise, the control is returned to the initial step for re-control.
Preferably, calculating the harmonic impedance according to the voltage, current and frequency data specifically includes:
selecting the frequency of disturbance harmonic waves;
injecting flying characteristic subharmonic current to the common coupling point, and carrying out real-time voltage and real-time current on the common coupling point;
and carrying out Fourier analysis on the real-time voltage and the real-time current, extracting harmonic quantity, and calculating harmonic impedance according to the harmonic quantity.
Preferably, the harmonic impedance ZhThe expression of(s) is:
Figure BDA0003410477410000021
in the above formula, s ═ jwh；wh＝2πfhThe angular frequency of the h harmonic is represented; f. ofhFrequency, U, of injection of disturbance harmonicsh(s)、IhAnd(s) represents the harmonic content of voltage and current.
Preferably, the maximum time limit allowed for normal operation of the frequency is 0.12 s.
Compared with the prior art, the invention has the following beneficial effects:
according to the virtual synchronous motor fault ride-through and safety protection control strategy provided by the invention, voltage, current and frequency data of a common coupling point PCC are collected; calculating harmonic impedance according to the voltage, current and frequency data; judging whether the harmonic impedance meets an island judgment condition, judging that an island phenomenon occurs when the harmonic impedance meets the island judgment condition, and entering island operation on the premise of allowing the island operation; otherwise, the phenomenon of non-island is adopted; when the non-isolated island phenomenon occurs, whether the grid fault ride-through FRT operates or not is judged according to the voltage and frequency data, when the FRT fails, if an isolated island system formed by the distributed power supply DER and the local load meets isolated island operation conditions, the isolated island operation can be started on the premise that the isolated island operation is allowed, and otherwise, the DER is directly disconnected. Effectively identifies island and grid fault ride-through, effectively coordinates the IP and FRT control of DER, and realizes safe and stable grid-connected operation of DER.
Drawings
In order to more clearly illustrate the technical solution of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only one embodiment of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.
FIG. 1 is a flow chart of a virtual synchronous machine fault ride-through and safety protection control strategy of the present invention;
FIG. 2 is a schematic diagram of a distributed inversion system according to the present invention;
FIG. 3 is a graph of the impedance Bode for a change in the resonant frequency of the load according to one embodiment of the present invention;
FIG. 4 is a graph of the impedance Bode as the load figure of merit is varied for one embodiment of the present invention;
FIG. 5 is a model of the harmonic transfer function of DIS in the grid-connected operating state of the present invention;
FIG. 6 is a harmonic circuit transfer function model of DIS in an island operation state of the present invention;
FIG. 7 is a harmonic circuit model of DIS in grid-connected operation of the present invention;
FIG. 8 is a harmonic circuit model of DIS in island operation of the present invention;
fig. 9 is a graph of the transient behavior of PCC harmonic impedance when the invention is islanded.
Detailed Description
The technical solutions in the present invention are 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.
As shown in fig. 1, the virtual synchronous machine fault-ride-through and safety protection control strategy provided by the present invention includes the following steps:
s1, collecting voltage, current and frequency data of the PCC;
distributed Inversion System (DIS) As shown in FIG. 2, there are two types of distributed inversion systemsPossible operating states: s1Closing and carrying out grid-connected operation; s1And (4) disconnecting and carrying out isolated island operation. The harmonic circuit models corresponding to the two operating states are shown in fig. 3 and 4, the corresponding transfer function models are shown in fig. 5 and 6, and the corresponding harmonic circuit models are shown in fig. 7 and 8. Wherein, IhA specific subharmonic current source at DIS side for detecting the change of the impedance value of PCC point; zoIs the equivalent output impedance of DIS; zlImpedance of a local load of RLC type; zgIs the equivalent impedance of the power grid; u shapepcc、IpccPCC point voltage and current values, respectively.
Zl(s) is the expression of the impedance of the RLC type local load:
Figure BDA0003410477410000041
Zg＝sLg+Rg (2)
in the above formula, R is the resistance of the local load, L is the inductance of the local load, s is a plurality, C is the capacitance of the local load, and ZgIs the equivalent impedance of the grid, LgEquivalent inductance of power grid, RgIs the equivalent resistance of the power grid.
Equivalent load impedance at PCC point seen by DIS side during grid-tie operation
Figure BDA0003410477410000042
Comprises the following steps:
Figure BDA0003410477410000043
during islanding operation, the equivalent load impedance at the PCC point, as seen from the DIS side
Figure BDA0003410477410000044
Comprises the following steps:
Figure BDA0003410477410000045
after the island is generated, the PCC equivalent impedance is seen from the DIS sidel//ZgBecome to ZlAnd Z isgUsually smaller, it can be determined whether islanding occurs based on the magnitude of the PCC equivalent impedance (harmonic impedance).
S2, calculating harmonic impedance Z according to the voltage, current and frequency datah；
As can be seen from fig. 3 and 4, in a certain frequency range, the PCC equivalent impedance has a large difference in the grid-connected and island operating states, and the impedance characteristics are substantially consistent for different local load characteristics.
According to the impedance characteristics, an uncharacteristic secondary frequency 75Hz which is closer to the fundamental frequency 50Hz is selected as the disturbance harmonic frequency, namely h is 1.5. Injecting non-characteristic subharmonic current (disturbance harmonic frequency) into the PCC point to carry out voltage and current U on the PCCpcc、IpccPerforming Fast Fourier Transform (FFT) analysis, and extracting corresponding harmonic amount Uh(s)、Ih(s) calculating the corresponding harmonic impedance Z according to the following formulah(s), harmonic impedance ZhThe expression of(s) is:
Figure BDA0003410477410000051
in the above formula, s ═ jwh；wh＝2πfhThe angular frequency of the h harmonic is represented; f. ofhFrequency, U, of injection of disturbance harmonicsh(s)、IhAnd(s) represents the harmonic content of voltage and current.
Wherein, FIG. 9 is Z before and after islandh(75Hz) simulation results, when operating normally grid-connected, ZhAbout 0.13 ohm; 1S, S1Disconnection, islanding, at which time ZhFrom 0.13ohm to about 0.25ohm and the value is kept constant after islanding has occurred.
S3, judging the harmonic impedance ZhWhether an island judgment condition is met or not, and when the island judgment condition is met, whether an island phenomenon occurs is judged, and on the premise that island operation is allowedThe lower part can enter into island operation; otherwise, the phenomenon of non-island is adopted;
the island judgment conditions include: the harmonic impedance is larger than the set impedance threshold, and the duration is longer than the maximum time limit T allowed by the normal operation of voltage and frequencyd1。
Wherein, the islanding detection comprises harmonic impedance type active islanding detection and improved OUV/OUF passive detection. The voltage transient disturbance phenomena such as voltage drop or rise can also cause the jump of harmonic impedance, thereby causing island misjudgment. However, the change in the harmonic impedance in this case is a transient phenomenon, and the duration is about 0.12s under the parametric simulation taken in this application. Therefore, under the two situations of the island and the transient voltage disturbance, the change of the PCC harmonic impedance has difference, and two different operation conditions can be distinguished according to the difference. Here, a certain delay is taken, when Z is detectedh>ZsetAnd a duration greater than Td1If not, the voltage transient disturbance phenomenon is judged, and the island protection action is not executed. Wherein an impedance threshold value Z is setsetTake 0.24ohm, Td1Take 0.12 s.
Theoretically, the PCC impedance does not change substantially when islanding does not occur. In practice, besides the islanding phenomenon, the harmonic impedance value is also changed due to disturbance phenomena such as grid voltage drop and the like, so that islanding misjudgment is caused. In order to effectively coordinate two functions of island protection and low-voltage ride through, transient disturbance phenomena such as grid voltage drop and the like need to be effectively identified so as to avoid misoperation of island protection. Various faults may occur in the operation process of the power grid, and voltage drop is one of the faults. The cause of voltage drop mainly refers to short-circuit faults of a power grid, and the short-circuit faults are divided into single-phase earth faults, two-phase earth faults, phase-to-phase faults and three-phase earth short-circuit faults.
S4, in practice, islanded operation of the I-DER is generally unsafe, and may or may cause personnel or equipment safety problems. Therefore, when the non-isolated island phenomenon occurs, whether the power grid fault ride-through FRT operates or not is judged according to the voltage and frequency data, when the FRT fails, if an isolated island system formed by the distributed power supply DER and the local load meets isolated island operation conditions, the isolated island operation can be started on the premise that the isolated island operation is allowed, and otherwise, the DER is directly disconnected.
Specifically, judging whether the power grid fault ride-through FRT operates according to the voltage and frequency data includes:
judging whether the voltage and the frequency are in a normal operation range, returning to the initial step for controlling again when the voltage and the frequency belong to the normal operation range, and judging whether to perform power grid fault ride-through when the voltage and the frequency do not belong to the normal operation range;
and when the voltage and the frequency reach the maximum time limit allowed by abnormal operation of the voltage and the frequency, if the voltage and the frequency still do not recover the normal operation range, the fault ride-through of the power grid fails, and otherwise, the control is returned to the initial step for re-control.
The voltage and frequency fluctuations created by the grid fault may cause the PCC voltage amplitude or frequency to exceed the normal operating range. According to the relevant islanding detection criteria, the conventional OUV/OUF passive detection method will quickly disconnect the DER from the grid once an out-of-limit condition is detected. In practice, some voltage fluctuations or frequency fluctuations are temporary and can generally be recovered to a normal operating range in a short time, and in such a case, the action of OUV/OUF protection can cause unnecessary disconnection of DER and even cause disconnection of other DER, thereby causing a power failure accident. To avoid this, the range of voltage and frequency protection is extended and its maximum operating time is adjusted in IEEE 1547a, see table 1. According to table 1, the action time of the OUV/OUF protection module is delayed to ensure that the fault ride-through is successful (only low voltage ride-through is considered here).
TABLE 1 island detection Standard specified in IEEE Std.1547a
Figure BDA0003410477410000061
Figure BDA0003410477410000071
Note: (1) u shapeNThe rated effective value of the amplitude of the power grid voltage is 220V.
(2)fNThe rated frequency of the single-phase alternating current is 50Hz, which is the rated frequency of the power grid voltage.
The above disclosure is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive the changes or modifications within the technical scope of the present invention, and shall be covered by the scope of the present invention.

Claims (6)

1. A virtual synchronous machine fault ride-through and safety protection control strategy is characterized by comprising the following steps:
s1, collecting voltage, current and frequency data of the PCC;
s2, calculating harmonic impedance according to the voltage, current and frequency data;
s3, judging whether the harmonic impedance meets an island judgment condition, judging that an island phenomenon occurs when the harmonic impedance meets the island judgment condition, and entering island operation on the premise of allowing the island operation; otherwise, the phenomenon is a non-isolated island phenomenon;
and S4, when a non-isolated island phenomenon occurs, judging whether the grid fault ride-through FRT operates according to the voltage and frequency data, when the FRT fails, if an isolated island system formed by the DER of the distributed power supply and the local load meets an isolated island operation condition, the device can enter the isolated island operation on the premise of allowing the isolated island operation, otherwise, the DER is directly disconnected.
2. The virtual synchronous machine fault ride-through and safety protection control strategy of claim 1, wherein the islanding determination condition comprises: the harmonic impedance is larger than a set impedance threshold, and the duration is longer than the maximum time limit allowed by normal operation of voltage and frequency.
3. The virtual synchronous machine fault ride-through and safety protection control strategy of claim 1, wherein judging whether a grid fault ride-through (FRT) is running according to the voltage and frequency data specifically comprises:
judging whether the voltage and the frequency are in a normal operation range, returning to the initial step for controlling again when the voltage and the frequency belong to the normal operation range, and judging whether to perform power grid fault ride-through when the voltage and the frequency do not belong to the normal operation range;
and when the voltage and the frequency reach the maximum time limit allowed by abnormal operation of the voltage and the frequency, if the voltage and the frequency still do not recover the normal operation range, the fault ride-through of the power grid fails, and otherwise, the control is returned to the initial step for re-control.
4. The virtual synchronous machine fault ride-through and safety protection control strategy of claim 1, wherein calculating harmonic impedance from the voltage, current and frequency data specifically comprises:
selecting the frequency of disturbance harmonic waves;
injecting flying characteristic subharmonic current to the common coupling point, and carrying out real-time voltage and real-time current on the common coupling point;
and carrying out Fourier analysis on the real-time voltage and the real-time current, extracting harmonic quantity, and calculating harmonic impedance according to the harmonic quantity.
5. The virtual synchronous machine fault ride-through and safety control strategy of claim 1, wherein the harmonic impedance ZhThe expression of(s) is:
Figure FDA0003410477400000021
in the above formula, s ═ jwh；wh＝2πfhThe angular frequency of the h harmonic is represented; f. ofhFrequency, U, of injection of disturbance harmonicsh(s)、IhAnd(s) represents the harmonic content of voltage and current.
6. The virtual synchronous machine fault ride-through and safety control strategy of claim 2, wherein the maximum time limit allowed for normal operation of the frequency is 0.12 s.
CN202111525939.6A 2021-12-14 2021-12-14 Virtual synchronous motor fault ride-through and safety protection control strategy Pending CN114421518A (en)

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