CN115882432A - Active injection type harmonic current differential protection method and system for power distribution network containing IIDG - Google Patents
Active injection type harmonic current differential protection method and system for power distribution network containing IIDG Download PDFInfo
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Abstract
The invention discloses an active injection type harmonic current differential protection method and system for a power distribution network containing an IIDG, and belongs to the technical field of relay protection of power systems. The method comprises the steps of acquiring current information and voltage information at an outlet of an inverter distributed power supply in real time; judging whether a starting criterion is met, if so, starting an injection unit, respectively injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter distributed power supply controller, and outputting characteristic harmonic currents by the inverter distributed power supply; and calculating the distortion rate difference value of the negative sequence component of the characteristic harmonic current on two sides of the protected line and the distortion rate difference value of the A-phase characteristic harmonic current according to the characteristic harmonic current, judging whether the differential protection criterion is met, and executing the protection action. The method has the advantages that fault characteristics are enhanced by using characteristic harmonic current injected into the power distribution network, differential protection of the power distribution network with the IIDG is achieved, and the problem that the setting and delay matching of traditional passive detection type protection are challenged due to uncertainty of IIDG output current is solved.
Description
Technical Field
The application relates to the technical field of relay protection of power systems, in particular to an active injection type harmonic current differential protection method and system for a power distribution network containing an IIDG.
Background
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
After a Distributed Generation (DG) is connected to a power distribution network in a large scale, the power distribution network is converted from a simple single-power radiation type power supply network into a complex multi-power multi-terminal power supply network, so that the characteristics of the power flow direction, the operation mode and the short-circuit current fault of the power distribution network containing the distributed generation are complex and changeable, and the traditional three-section type current protection is not suitable any more.
Depending on the grid-connected interface, the DG may be classified into a direct grid-connected rotary distributed generator (MTDG) and an inverter-type distributed generator (IIDG) that is grid-connected via an inverter. Due to low inertia, high controllability and poor overcurrent capacity of the power electronic inverter, the IIDG has fault characteristics different from those of a traditional synchronous generator, in order to prevent overcurrent damage of power electronic devices of the inverter, the maximum short-circuit current provided by the IIDG is limited within 2 times of rated current, and the output current of the IIDG is influenced by factors such as inverter control strategies, low voltage ride through and fault conditions.
In order to solve the problems, experts and scholars at home and abroad apply current differential protection to a power distribution network, but the phase of the output current of the IIDG is changed greatly after a fault occurs, so that the traditional current differential protection is possibly refused to operate under a certain fault scene, and the traditional current longitudinal differential protection has a high requirement on communication synchronization. In conclusion, the uncertainty of the IIDG output current makes the setting and delay matching of the traditional passive detection type protection face a great challenge, especially for a power distribution network with high IIDG permeability. As IIDG permeability increases over time, a new protection approach is needed.
Disclosure of Invention
In order to overcome the defects of the prior art, the application provides an active injection type harmonic current differential protection method and system for a power distribution network containing an IIDG, and the characteristic harmonic current injected into the power distribution network is utilized to strengthen the fault characteristic, so that the differential protection of the power distribution network containing the IIDG is realized.
In a first aspect, the application provides an active injection type harmonic current differential protection method for a power distribution network containing an IIDG;
an active injection type harmonic current differential protection method for a power distribution network containing an IIDG comprises the following steps:
s1, current information and voltage information at an outlet of an inversion distributed power supply are obtained in real time;
s2, judging whether the starting criterion is met or not according to the current information and the voltage information at the outlet of the inversion distributed power supply, and if so, continuing to execute the following steps; if not, returning to execute the step S1;
s3, starting an injection unit, injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter distributed power supply controller respectively, and outputting characteristic harmonic currents by the inverter distributed power supply;
and S4, calculating the distortion rate difference value of the negative sequence component of the characteristic harmonic current on the two sides of the protected line and the distortion rate difference value of the A-phase characteristic harmonic current on the two sides of the protected line according to the characteristic harmonic current, judging whether a differential protection criterion is met, and if so, executing a protection action.
Further, the starting criteria include a phase voltage starting criterion and a phase current starting criterion, wherein the phase current starting criterion is expressed as:
in the formula ,
for the number of sampling points corresponding to the current time, is greater than or equal to>
Is the number of sampling points of a cycle, is greater than or equal to>
Is the number of sampling points of the previous cycle at the current moment, is greater than or equal to>
The number of sampling points of two cycles before the current time is greater or less>
For the value of the sampling of the phase current at the present time>
For the sampling value of the phase current of the cycle preceding the present time, in the on/off position>
For the sampled value of the phase current of the two cycles preceding the present moment, is->
A start coefficient and a value of 0.1 to 0.3, based on the measured values>
Is the rated current of the protected feeder.
Further, step S1 further includes:
if the start-up criterion is met, an enable signal is sent to the injection unit to start the injection unit.
Further, the characteristic harmonic current of the a phase is expressed as:
wherein ,
is the amplitude of the characteristic harmonic current>
Is the phase angle of a characteristic harmonic signal>
Is the phase angle of power frequency>
Is the angular frequency of the characteristic harmonic signal>
Is the angular frequency of the power frequency>
Is the current moment, is greater or less than>
For the frequency of a characteristic harmonic signal injected into a d-axis, q-axis reference current>
At power frequency>
Is the initial phase angle of the characteristic harmonic signal.
Further, the amplitude a of the characteristic harmonic current is expressed as
wherein ,IIIDG_d Is a d-axis reference current, I IIDG_q Is a q-axis reference current.
Further, the calculation of the difference value of the distortion rates of the negative sequence components of the characteristic harmonic currents on the two sides of the protected line is as follows:
respectively calculating negative sequence components in short-circuit currents on two sides of a protected line, wherein the negative sequence components are negative sequence components of characteristic harmonic currents;
respectively calculating the distortion rate of the negative sequence component of the characteristic harmonic current according to the negative sequence component of the characteristic harmonic current and the negative sequence component of the power frequency current;
calculating the absolute value of the difference in the negative sequence component distortion rates;
the calculation of the difference value of the A-phase characteristic harmonic current distortion rates of the two sides of the protected line is as follows:
respectively calculating A-phase characteristic harmonic current distortion rates at two sides of the protected line according to the amplitude of the A-phase characteristic harmonic current and the amplitude of the A-phase power frequency current;
the absolute value of the difference in the a-phase characteristic harmonic current distortion rate is calculated. Further, the judging whether the differential protection criterion is satisfied is as follows:
judging whether the absolute value of the difference value of the negative sequence component distortion rate of the characteristic harmonic current in the short-circuit current on the two sides of the protected line is greater than a first differential protection action threshold value or not;
judging whether the absolute value of the difference value of the A-phase characteristic harmonic current distortion rate in the short-circuit current on the two sides of the protected line is greater than a second differential protection action threshold value or not;
if the difference value is greater than the first differential protection action threshold value or greater than the second differential protection action threshold value, executing differential protection action and stopping the injection of the characteristic harmonic signal;
the first differential protection action threshold and the second differential protection action threshold are equal or unequal.
Further, the calculation of the distortion rate of the negative sequence component of the characteristic harmonic current is expressed as follows:
wherein h is the number of characteristic harmonics,
is the amplitude of the negative sequence component of the h-th characteristic harmonic current>
The amplitude of the negative sequence component of the power frequency current.
Furthermore, the frequency of the characteristic harmonic current is not more than 10 times of the power frequency and is an integral multiple of the power frequency.
In a second aspect, the application provides an active injection type harmonic current differential protection system for a power distribution network containing an IIDG;
an active injection type harmonic current differential protection system for a power distribution network containing an IIDG comprises:
a fault detection and harmonic injection control unit configured to: acquiring current information and voltage information at an outlet of the inverter distributed power supply in real time; judging whether a starting condition is met or not according to current information and voltage information at an outlet of the inversion distributed power supply, and if so, starting an injection unit;
an injection unit configured to: injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter distributed power supply controller, and outputting characteristic harmonic currents by the inverter distributed power supply;
a differential protection unit configured to: and calculating the distortion rate difference value of the negative sequence component of the characteristic harmonic current on the two sides of the protected line and the distortion rate difference value of the A-phase characteristic harmonic current on the two sides of the protected line according to the characteristic harmonic current, judging whether the differential protection criterion is met, and if so, executing a protection action.
Compared with the prior art, the beneficial effect of this application is:
1. according to the technical scheme, the characteristic harmonic current amplitude injected into the power distribution network is changed along with the IIDG output current amplitude in a self-adaptive mode, the fault characteristics of the characteristic harmonic current after injection are enhanced through the self-adaptive change, and meanwhile the condition that the harmonic injection rule of the power distribution network can be met all the time can be guaranteed.
2. According to the technical scheme, the harmonic distortion rate is used for constructing the differential protection criterion, so that the interference of a large amount of harmonic waves generated at the moment of a fault on protection can be reduced.
3. According to the technical scheme provided by the application, the active injection type harmonic current differential protection method is simple in action threshold setting, has excellent selectivity and sensitivity, does not need strict communication synchronization requirements, and is strong in transition resistance tolerance.
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The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the application and, together with the description, serve to explain the application and are not intended to limit the application.
Fig. 1 is a schematic diagram illustrating a start detection principle of an IIDG injection unit according to an embodiment of the present application;
fig. 2 is a schematic diagram of a simple active power distribution network including an IIDG access according to an embodiment of the present application;
fig. 3 is a schematic structural diagram of an IIDG overall control system provided in the embodiment of the present application;
fig. 4 is a schematic flowchart of an active injection type harmonic current differential protection method for a power distribution network including an IIDG according to an embodiment of the present application;
fig. 5 is a schematic circuit diagram of an IIDG1 outlet circuit of the simple active power distribution network including the IIDG according to the embodiment of the present application when the power distribution network operates normally;
fig. 6 is a schematic diagram of a characteristic harmonic current amplitude and a characteristic harmonic signal waveform injected into d and q axes when the simple active power distribution network including the IIDG according to the embodiment of the present application operates normally;
fig. 7 is a schematic diagram of characteristic harmonic current distortion at an outlet of an IIDG1 in a normal operation of a simple active power distribution network including an IIDG according to an embodiment of the present application;
fig. 8 is a schematic current diagram at an outlet of the IIDG1 when the simple active power distribution network including the IIDG provided in the embodiment of the present application fails;
fig. 9 is a schematic diagram of a characteristic harmonic current amplitude and a characteristic harmonic signal waveform injected into d and q axes when an IIDG-containing simple active power distribution network provided by an embodiment of the present application is in a fault;
fig. 10 shows characteristic harmonic current distortion rates at an outlet of the IIDG1 when the simple active power distribution network including the IIDG provided in the embodiment of the present application fails;
fig. 11 (a) is a schematic diagram of distortion rates of characteristic harmonic current negative sequence components at two sides of a feeder line BM when a single-phase ground is shorted and when an f1 point asymmetric fault of a simple active power distribution network including an IIDG and a transition resistance are 0.1 Ω, provided in the embodiment of the present application;
fig. 11 (b) is a schematic diagram of distortion rates of characteristic harmonic currents negative sequence components on two sides of a feeder BM when two phases are grounded and shorted when an asymmetric fault of a point f1 of a simple active power distribution network including an IIDG and a transition resistance is 0.1 Ω, provided in the embodiment of the present application;
fig. 11 (c) is a schematic diagram of distortion rates of characteristic harmonic current negative sequence components at two sides of a feeder line BM when two phases are short-circuited in a simple active power distribution network including an IIDG provided in the embodiment of the present application, where the f1 point of the simple active power distribution network includes an asymmetric fault and a transition resistance is 0.1 Ω;
fig. 12 is a diagram illustrating f in a simple active power distribution network including IIDG according to an embodiment of the present application 1 Point symmetry failure, when the transition resistance is 0.1 omega, the feeder line BM is twoA side A phase characteristic harmonic current distortion rate diagram;
fig. 13 (a) shows f in a simple active power distribution network including IIDG according to an embodiment of the present application 1 Point asymmetry fault, 20 omega transition resistance, characteristic harmonic current negative sequence component distortion diagram on two sides of feeder BM during single phase grounding short circuit;
fig. 13 (b) shows f in a simple active power distribution network including IIDG according to an embodiment of the present application 1 Point asymmetry fault, transition resistance 20 omega, feeder line BM both sides characteristic harmonic current negative sequence component distortion rate schematic diagram when two-phase ground short circuit;
fig. 13 (c) shows f in a simple active power distribution network including an IIDG according to an embodiment of the present application 1 Point asymmetry fault, transition resistance 20 omega, feeder BM both sides characteristic harmonic current negative sequence component distortion rate schematic diagram when two-phase short circuit.
Fig. 14 shows f in a simple active power distribution network including IIDG according to an embodiment of the present application 1 Point symmetry faults, when the transition resistance is 20 omega, a schematic diagram of A-phase characteristic harmonic current distortion rates on two sides of a feeder line BM;
fig. 15 (a) shows a simple active power distribution network f including IIDG according to an embodiment of the present application 2 Point asymmetry fault, when the transition resistance is 0.1 omega, the distortion rate of the characteristic harmonic current negative sequence component at two sides of the single-phase grounding short circuit feeder MN is schematic;
fig. 15 (b) shows f in a simple active power distribution network including an IIDG according to an embodiment of the present application 2 Point asymmetry fault, when the transition resistance is 0.1 omega, the distortion rate of the characteristic harmonic current negative sequence component at two sides of the two-phase grounding short circuit feeder MN is schematic;
fig. 15 (c) shows a simple active power distribution network f including IIDG according to an embodiment of the present application 2 Point asymmetry fault, when the transition resistance is 0.1 omega, the distortion rate of the characteristic harmonic current negative sequence component at two sides of the two-phase short circuit feeder MN is schematic;
fig. 16 shows f in a simple active power distribution network including IIDG according to an embodiment of the present application 2 Point symmetry faults, namely when the transition resistance is 0.1 omega, the distortion rate of A-phase characteristic harmonic currents on two sides of the feeder line MN;
fig. 17 (a) shows a simple active power distribution network f including IIDG according to an embodiment of the present application 2 Point asymmetry fault, transition resistanceWhen the voltage is 20 omega, the distortion rate of the characteristic harmonic current negative sequence components on two sides of the feeder line MN is schematic when the single-phase grounding short circuit occurs;
fig. 17 (b) shows f in a simple active power distribution network including IIDG according to an embodiment of the present application 2 Point asymmetry fault, 20 omega transition resistance, and a characteristic harmonic current negative sequence component distortion rate schematic diagram at two sides of a feeder MN during two-phase grounding short circuit;
fig. 17 (c) shows f in a simple active power distribution network including an IIDG according to an embodiment of the present application 2 Point asymmetry fault, when the transition resistance is 20 omega, the distortion rate of characteristic harmonic current negative sequence components at two sides of the feeder MN is schematic when two phases are short-circuited;
FIG. 18 shows a simple active distribution network f including IIDGs according to an embodiment of the present disclosure 2 And when point symmetry faults occur and the transition resistance is 20 omega, the characteristic harmonic current distortion rate of the A phases on two sides of the feeder line MN is schematic.
In the figure: a. positive and negative sequence separation links; b. a constant power control link based on the positive sequence component; c, restraining a control link of negative sequence current; d. a low voltage ride through control link; e. and (4) fault detection and harmonic injection control links.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure herein. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, and it should be understood that the terms "comprises" and "comprising", and any variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
The embodiments and features of the embodiments of the present invention may be combined with each other without conflict.
Example one
In the prior art, the uncertainty of the IIDG output current causes great challenge to the setting and delay matching of the traditional passive detection type protection; therefore, the application provides an active injection type harmonic current differential protection method for a power distribution network containing an IIDG.
Next, an active injection type harmonic current differential protection method for a power distribution network including an IIDG disclosed in this embodiment is described in detail with reference to fig. 1 to 18. The active injection type harmonic current differential protection method for the power distribution network containing the IIDG comprises the following steps:
the method comprises the steps of S1, acquiring current information and voltage information at an outlet of an inversion distributed power supply (IIDG) in real time, wherein the current information is acquired by a current transformer installed at the outlet of the inversion distributed power supply (IIDG), and the voltage information is acquired by a voltage transformer installed at the outlet of the inversion distributed power supply (IIDG).
S2, judging whether a starting judgment result is met or not to start the injection unit according to current information and voltage information at an outlet of an inversion distributed power supply (IIDG), and if so, continuing to execute the following steps; if not, returning to execute the step S1;
specifically, in order to avoid that the characteristic harmonic current injected under the normal operation condition of the power distribution network harms users, the embodiment designs a corresponding starting criterion, judges whether to start the injection unit according to whether a fault occurs, and starts to inject the characteristic harmonic current only after the fault occurs.
For example, in this embodiment, the IIDG injection unit start detection principle is as shown in fig. 1, a fault is detected by collecting a voltage or current abrupt change at an IIDG outlet, and when a start criterion is met, an enable signal is injected to the IIDG controller internal injection unit.
The fault detection according to the current break variable at the IIDG outlet (i.e., the phase current break variable fault detection algorithm in fig. 1) is represented as:
wherein ,
for the number of sampling points corresponding to the current time, is greater than or equal to>
Number of samples taken for a cycle>
Is the number of sampling points of the previous cycle at the current moment, is greater than or equal to>
The number of sampling points of two cycles before the current time is greater or less>
For the sampled value of the phase current at the present time, is->
For the sampling value of the phase current of the cycle preceding the present time, in the on/off position>
For the sampled value of the phase current of the two cycles preceding the present moment, is->
Is a start coefficient and takes a value of 0.1 to 0.3->
Is the rated current of the protected feeder.
The fault detection according to the voltage break variable at the IIDG outlet is represented as:
in the formula ,
is the current timeThe corresponding sampling point number is judged>
Number of samples taken for a cycle>
A number of samples taken in a cycle preceding the current time>
Is the sampling point number of the previous two cycles at the current moment, is counted and then is judged>
For the value sampled at the phase voltage at the present time>
For the sampling value of the phase voltage of the cycle preceding the present time instant, is->
For the sampling value of the two cycle phase voltages preceding the present time>
The starting coefficient is 0.1 to 0.3, and the value is the rated voltage of the protected feeder line.
S3, starting an injection unit, injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter type distributed power supply (IIDG) controller respectively, wherein the amplitude of the characteristic harmonic signals changes along with the amplitude of output current of the inverter type distributed power supply (IIDG) in a self-adaptive mode, and injection of characteristic harmonic current is completed;
wherein, the power frequency current amplitude value of the IIDG output =
；
in the formula ,IIIDG_d Is a d-axis reference current, I IIDG_q Is a q-axis reference current.
Specifically, the IIDG for grid-connected operation in the actual power distribution network mostly adopts a constant Power (PQ) control strategy for inhibiting negative sequence components and has low voltage ride through capability, and an IIDG control systemThe system is a double-ring control structure, the outer ring is a power ring, and the inner ring is a current ring. At the moment after the power distribution network fails, characteristic harmonic signals are supposed to be respectively injected into d-axis reference current and q-axis reference current of the current inner ring
According to Park conversion, the instantaneous value of the three-phase current output by the IIDG after injection can be obtained as
wherein ,
，/>
，/>
for instantaneous value of phase-A current>
For the instantaneous value of phase B current>
For the instantaneous value of phase C current>
Angular frequency for characteristic harmonic signals>
Is the angular frequency of the power frequency>
Is the corresponding phase angle at time t of the characteristic harmonic signal>
Is the corresponding phase angle of the power frequency at the time t->
For injecting the amplitude of the characteristic harmonic signal, I IIDG_d Is a d-axis reference current, I IIDG_q Is a q-axis reference current.
From the above formula, the current I is referred to the d-axis and q-axis respectively IIDG_d 、I IIDG_q After the characteristic harmonic signal is injected, a new characteristic harmonic current exists in the IIDG output current, the new characteristic harmonic current is three-phase symmetrical, analysis is carried out by taking the A-phase output current as an example, and the new A-phase characteristic harmonic current is
wherein ,
is the amplitude of the characteristic harmonic current, is greater than or equal to>
Is the phase angle of a characteristic harmonic signal>
Is the phase angle of power frequency>
Is the angular frequency of the characteristic harmonic signal>
Is the angular frequency of the power frequency>
Is the current moment, is greater or less than>
For the frequency of a characteristic harmonic signal injected into the d-axis, q-axis reference current, <' > or>
Is power frequency and is greater or less than>
Is the initial phase angle of the characteristic harmonic signal.
The amplitude of the new A-phase characteristic harmonic current is
At a frequency of->
Hz。
aIs shown as
wherein ,IIIDG_d Is a d-axis reference current, I IIDG_q Is a q-axis reference current.
For the selection of the characteristic harmonic current frequency, the characteristic frequency selection is very important and cannot be selected randomly, and the following constraints need to be considered.
1) Characteristic frequency not more than 10 times power frequency
The resonance frequency of the IIDG filter is generally and widely distributed in the range from 10 times of power frequency to 0.5 time of carrier frequency, the characteristic frequency is selected according to the constraint 1), the characteristic harmonic current and the IIDG filter can be prevented from generating resonance, and therefore the IIDG can be ensured to operate safely and stably. In order to suppress the shunting effect of the non-injected source IIDG on the characteristic harmonic current, the characteristic frequency cannot be much smaller than the resonant frequency.
2) The characteristic frequency should be as far away from the power frequency as possible
When the power distribution network is disturbed or has faults, harmonic waves are inevitably generated and are mainly low-order harmonic waves. If the characteristic frequency is far away from the power frequency, the interference of the generated harmonic waves on the injection protection can be reduced as much as possible.
3) The characteristic frequency should be an integral multiple of the power frequency
The integral multiple of the power frequency is selected, so that the extraction, analysis and processing of general time frequency analysis methods such as fast Fourier transform, wavelet transform and the like are facilitated, the signal processing delay is shortened, and the engineering practicability of the injection type protection method is improved.
For the selection of the characteristic harmonic current amplitude, according to the latest regulation, when the grid-connected inverter operates at rated power, the limit value of the total harmonic distortion of the current injected into the power grid is 5%, so that the amplitude of the injected characteristic harmonic current needs to be less than 5% of the output current of the grid-connected inverter.
In summary, in this embodiment, if 8 th harmonic (frequency 400 Hz) signals are injected into the d-axis and q-axis reference currents of the IIDG inverter control system, 7 th harmonic (frequency 350 Hz) currents exist in the IIDG output current. Under normal operation or various fault scenes, the amplitude of the injected harmonic wave is always kept to be 4% of the amplitude of the output current of the grid-connected inverter.
And S4, calculating a distortion rate difference value of the characteristic harmonic current negative sequence components at two sides of the protected line and an A-phase characteristic harmonic current distortion rate difference value, judging whether a differential protection criterion is met, and if so, executing a protection action.
Specifically, three-phase reclosing is mostly adopted in the power distribution network, the fault type does not need to be judged, in order to reduce the data volume of communication transmission, a sequence component is preferentially adopted in the embodiment to construct a protection criterion, and positive, negative and zero sequence decomposition is carried out on a characteristic harmonic current component in the short-circuit current on the side of the IIDG, as shown in the following formula:
wherein ,
for a zero-sequence current component of the characteristic harmonic current, based on the comparison of the characteristic harmonic current>
For the zero-sequence current component of the characteristic harmonic current, is greater than or equal to>
Is a negative-sequence current component of the characteristic harmonic current, is greater than or equal to>
Is an initial phase angle of a characteristic harmonic signal>
Angular frequency for characteristic harmonic signals>
Is the current time。
According to the formula, the characteristic harmonic current only contains a negative sequence component and does not contain a positive sequence component and a zero sequence component after positive, negative and zero sequence decomposition.
Taking the simple active power distribution network with IIDG access in fig. 2 as an example, the negative sequence component of the characteristic harmonic current in the short-circuit current on both sides of the protected feeder line section is analyzed, in fig. 2, L 1 、L 2 、L 3 、L 4 As a load, K 1 、K 2 、K 3 、K 4 As a circuit breaker, f 1 、f 2 Is a point of failure.
Suppose f 1 After the point is in fault, the short-circuit current provided by the system power supply and the short-circuit current provided by the DG both flow to the fault point, the short-circuit current provided by the DG contains the injected characteristic harmonic current component, and the short-circuit current provided by the system power supply does not contain or less contains the characteristic harmonic component. Therefore, according to the magnitude of the characteristic harmonic current negative sequence component in the short-circuit current on two sides of the protected section BM, fault positioning and cutting can be realized. Meanwhile, in order to reduce the interference of a large amount of harmonics generated at the moment of a fault on protection, the differential protection criterion is constructed on the basis of the distortion rate of the negative sequence component of the characteristic harmonic current.
Calculating the distortion rate of the negative sequence component of the characteristic harmonic current:
wherein h is the number of characteristic harmonics,
is the magnitude of the negative sequence component of the h-th harmonic current, is preset>
The amplitude of the negative sequence component of the power frequency current.
And (3) calculating the distortion rate of the A-phase characteristic harmonic current:
wherein h is the number of characteristic harmonics,
is the amplitude of the h-th harmonic current of phase A>
The amplitude of the A-phase power frequency current is obtained.
When an asymmetric fault occurs, the negative sequence component of the power frequency current provided by the system power supply is large, and the distortion rate of the negative sequence component of the characteristic harmonic current on the system side can be further weakened as can be seen from the formula, and because the IIDG adopts a control strategy for inhibiting the negative sequence current, the output power frequency negative sequence current is small, so that the distortion rate of the negative sequence component of the characteristic harmonic current on the IIDG side of the feeder line is large, and based on the difference value of the distortion rates of the negative sequence components of the characteristic harmonic currents on the two sides of the feeder line, the differential protection of the feeder line can be realized.
When a symmetric fault occurs, the negative sequence component of the power frequency current is about 0 in a short time after the fault, and at the moment, the characteristic harmonic current negative sequence component distortion rate differential protection criterion is not applicable. The difference value of the A-phase characteristic harmonic current distortion rates on the two sides of the protected feeder line is used as an auxiliary criterion, so that the problem of dead zones of the characteristic harmonic current negative sequence component distortion rate differential protection criterion during three-phase short circuit can be effectively solved.
In summary, the characteristic harmonic current differential protection criterion is
1) At asymmetric fault
wherein ,
for the characteristic harmonic current negative sequence component distortion rate on the power supply side of the protected feeder system, is/are->
For the distortion rate of the negative sequence component of the characteristic harmonic current on the distributed power supply side, <' >>
Is a first differential protection action threshold.
2) At the time of symmetrical failure
wherein ,
for the characteristic harmonic current distortion rate of the A phase on the power supply side of the protected feeder system, in each case>
For the characteristic harmonic current distortion rate of the A phase on the distributed power supply side->
Is a second differential protection action threshold.
Specifically, the threshold of the differential protection action is generally 0.3-0.5 times of the total harmonic current distortion at the IIDG outlet, and in this embodiment, the first differential protection action threshold and the second differential protection action threshold are equal to each other and are 0.4 times of the total harmonic current distortion at the IIDG outlet.
Fig. 3 is a diagram of the IIDG overall control system, wherein the letter numbers in the upper right corner of each module represent the meanings: in the positive and negative sequence separation step a, positive and negative sequence separation of the electric quantity is realized by adopting a signal delay method; in a constant power control link b based on the positive sequence component, the outer ring is used for power control, and the inner ring is used for current control; a control link c for inhibiting negative sequence current; a low voltage ride through control link d; and e, controlling a link e by fault detection and harmonic injection. When the power distribution network has a fault, a fault detection and harmonic injection control link e in the IIDG control system detects the fault, and an enabling signal is injected into a constant power control link b based on a positive sequence component; after receiving the enabling signal, the constant power control link b based on the positive sequence component injects characteristic harmonic current into d-axis reference current and q-axis reference current of the current inner loop control link respectively, the amplitude of the characteristic harmonic current changes along with the amplitude of IIDG output current in a self-adaptive mode and is always kept to be 4% of the amplitude of the output current, the fault characteristic of the characteristic harmonic current after injection is strengthened, meanwhile, grid connection regulation can be still met under the fault condition, and finally, characteristic signal injection is completed. The injection type harmonic current differential protection implementation process of the power distribution network with the IIDG is shown in fig. 4.
In order to verify the feasibility of the active injection type harmonic current differential protection method for the power distribution network containing the IIDG, a 10kV simple active power distribution network model containing the IIDG, which is shown in FIG. 2, is built by using PSCAD/EMTDC simulation software, wherein the lengths of feeder lines BM and MN are both 4km, and the line parameters are (0.13 + i0.402) omega/km; DG1 and DG2 are inversion distributed power supplies, and the rated capacities are both 1MW; the load capacity is 1MW, and the load power factor is 0.9.
1) Verifying correctness of characteristic harmonic current injection
Example 1: normal operation, 0.3s injection of characteristic harmonics
The power distribution network with the IIDG normally operates, the IIDG1 and the IIDG2 manually start to inject characteristic harmonic current according to an injection strategy at 0.3s, current waveforms at an outlet of the IIDG1 before and after injection are shown in a figure 5, amplitude values of phase-A power frequency current and the characteristic harmonic current are extracted by using fast Fourier transform, and characteristic harmonic current distortion rate at the outlet of the IIDG1 is calculated. The amplitude of the characteristic harmonic current of the phase A at the outlet of the IIDG1 and the waveform of the characteristic harmonic signal injected into the d axis and the q axis are shown in figure 6, and the distortion rate of the characteristic harmonic current at the outlet is shown in figure 7.
As can be seen from fig. 5, there is a characteristic harmonic component in the current at the IIDG1 outlet after 0.3 s. Due to the inherent time delay of the fast Fourier transform operation, the amplitude of the extracted characteristic harmonic current reaches stability after one cycle, and the stable value is approximately equal to the amplitude of the characteristic harmonic signal injected into the current inner loop to control the d and q axes, as shown in FIG. 6. As shown in fig. 7, the distortion rate of the characteristic harmonic current at the IIDG outlet stabilizes to 4% after one cycle, and the simulation result is consistent with the theoretical analysis, so that the correctness of the characteristic harmonic injection system under the normal operation condition can be verified.
Example 2: f. of 2 Point AB interphase short circuit fault, transition resistance 0.1 omega
time t =0.3sf 2 When AB interphase short circuit fault occurs at a point, the transition resistance of the fault point is 0.1 omega, and IIDG1 and IIDG2 start according to an injection strategy after detecting the faultA characteristic harmonic current is injected. The current waveforms at the outlet of the IIDG1 before and after the fault are shown in fig. 8, the amplitude of the phase a characteristic harmonic current at the outlet of the IIDG1 and the waveforms of characteristic harmonic signals injected into the d and q axes are shown in fig. 9, and the distortion of the characteristic harmonic current is shown in fig. 10.
The rated current of the maximum output short-circuit current of the IIDG is set to be 1.5 times, and as can be seen from figure 8, the IIDG1 carries out low-voltage ride through at 0.3s, the current at an outlet is increased to be 1.5 times of the original current, and the output current after the fault is still three-phase symmetrical because the IIDG adopts a control strategy of inhibiting negative sequence current. Due to the inherent delay of the fast fourier transform operation, the amplitude of the characteristic harmonic current after the fault is stable after one cycle, and the stable value is approximately equal to the amplitude of the characteristic harmonic signal injected into the current inner loop to control the d and q axes, as shown in fig. 9. As shown in fig. 10, the characteristic harmonic current distortion at the IIDG outlet is stabilized to 4% after one cycle, and it can be known that the characteristic harmonic current amplitude after the fault is always 0.04 times of the IIDG output current, and the above simulation result is consistent with the above theoretical analysis, so that the correctness of the characteristic harmonic injection system under the fault condition can be verified.
2) Verifying correctness of characteristic harmonic current differential protection method
Example 1: f. of 1 Point asymmetric short circuit fault, transition resistance 0.1 omega
Let t =0.3s time f 1 And when the point has single-phase grounding, two-phase grounding and two-phase short circuit faults, the transition resistance of the fault point is 0.1 omega, and after the IIDG1 and the IIDG2 detect the faults, the injection of characteristic harmonic current is started according to an injection strategy. The distortion rate of the characteristic harmonic current negative sequence component on both sides of the feeder line BM after the fault is shown in fig. 11 (a) -11 (c).
Fig. 11 (a) -11 (c): I.C. A jm 、I jb And respectively feeding line BM distributed power supply side and system power supply side characteristic harmonic current negative sequence component distortion rates. The characteristic harmonic current negative sequence component distortion rate differential protection action threshold is set to be 2%. As can be seen from fig. 11 (a) -11 (c), when a single-phase ground fault occurs in the feeder BM region, the difference between the distortion rates of the two sides of the feeder is about 0.45 after a cycle, and the difference between the distortion rates under a two-phase ground fault and a two-phase short fault is about 0.06 after a cycle, which, in summary,f 1 the characteristic harmonic current differential protection can reliably act under various asymmetric faults.
Example 2: f. of 1 Point-symmetrical short-circuit fault with 0.1 omega transition resistance
On the basis of example 1, f is 1 The point fault type is changed into a three-phase short-circuit fault, the transition resistance of the fault point is 0.1 omega, and the distortion rate of the A-phase characteristic harmonic current at two sides of the feeder line BM after the fault is shown in figure 12.
In fig. 12: i is jma 、I jba And respectively feeding line BM distributed power supply side and system power supply side A-phase characteristic harmonic current distortion rate. The characteristic harmonic current distortion rate differential protection action threshold is set to 2%. As can be seen from FIG. 12, the difference value of the A-phase characteristic harmonic current distortion on the two sides of the feed line is about 4% after being stabilized, which is consistent with the theoretical analysis, therefore, f 1 The characteristic harmonic current differential protection can reliably act under the point-symmetric fault.
Example 3: f. of 1 Point asymmetric short circuit fault, transition resistance 20 omega
On the basis of example 1, f is 1 And the point transition resistance is changed to 20 omega, and after the IIDG1 and the IIDG2 detect the fault, the injection of the characteristic harmonic current is started according to an injection strategy. The distortion rate of the characteristic harmonic current negative sequence component on both sides of the feeder line BM after the fault is shown in fig. 13 (a) -13 (c).
As can be seen from fig. 13 (a) -13 (c), when a single-phase ground fault occurs in the feeder BM region, the distortion rate difference value at both sides of the feeder is about 1.58 after one cycle, the distortion rate difference value under two-phase ground and two-phase short circuit faults is about 1.00 and 0.41 after one cycle, and the distortion rate difference value under high-resistance asymmetric fault is far greater than the action threshold, which is because the large transition resistance at the fault point causes the small power frequency negative sequence component at the distributed power supply side, so that the distortion rate of the characteristic harmonic current negative sequence component at the distributed power supply side is large, and in conclusion, the characteristic harmonic current differential protection under various asymmetric high-resistance faults at the f1 point can reliably act, and has high sensitivity.
Example 4: f. of 1 Point-symmetrical short-circuit fault with transition resistance of 20 omega
On the basis of example 2, f is 1 The point-fault transition resistance is changed to 20 omega,the characteristic harmonic current distortion of the a-phase on both sides of the feeder BM after the fault is shown in fig. 14.
As can be seen from fig. 14, the difference value of the distortion rate of the characteristic harmonic current of the a-phase on both sides of the feeder line is about 3.8% and slightly lower than 4% after being stabilized, which is because the transition resistance of the fault point is large, so that most of the characteristic harmonic current flows to the fault point, and the other small part of the characteristic harmonic current flows to the system power supply downstream of the fault point. In summary, f 1 The characteristic harmonic current differential protection under the point high-resistance symmetrical fault can reliably act.
Example 5: f. of 2 Point asymmetric short circuit fault, transition resistance 0.1 omega
On the basis of the formula 1, the fault point is changed to f 2 And point, the transition resistance of the fault point is 0.1 omega, and IIDG1 and IIDG2 start to inject characteristic harmonic current according to an injection strategy. The distortion rate of the characteristic harmonic current negative sequence component on both sides of the feeder MN after the fault is shown in fig. 15 (a) -15 (c).
Fig. 15 (a) -15 (c): i is jm 、I jn And respectively feeding characteristic harmonic current negative sequence component distortion rates of the MN system power supply side and the distributed power supply side. As can be seen from fig. 15 (a) -15 (c), when a single-phase ground fault occurs in the MN area of the feeder, the difference between the distortion rates of the two sides of the feeder is about 0.18 after a cycle, and the difference between the distortion rates under the two-phase ground fault and the two-phase short circuit fault is about 0.06 and 0.055 after a cycle. In addition, it should be noted that part of the characteristic harmonic current provided by the IIDG1 flows into the fault point, and the other part of the characteristic harmonic current flows into the system power supply side, but the characteristic harmonic current flowing into the fault point is far smaller than the short-circuit current provided by the system power supply, and the influence on the differential protection of the characteristic harmonic current of the feeder MN can be ignored.
Example 6: f. of 2 Point-symmetric short-circuit fault with transition resistance of 0.1 omega
On the basis of the calculation example 2, the fault point is changed intof 2 At the point, the transition resistance of the fault point is 0.1 Ω, and the distortion rate of the characteristic harmonic current of the a-phase at the two sides of the feeder MN after the fault is shown in fig. 16.
In fig. 16: i is jma 、I jna And respectively feeding line MN system power supply side and distributed power supply side A-phase characteristic harmonic current distortion rate. As can be seen from fig. 16, the difference value of the distortion rate of the characteristic harmonic current of the phase a on both sides of the feeder MN is about 4% after being stabilized, and the correctness of the inference of "neglecting the influence of the characteristic harmonic current injected by the IIDG1 on the differential protection of the characteristic harmonic current of the feeder MN" in the calculation example 5 is further verified, therefore, f 2 The characteristic harmonic current differential protection under the point-symmetric fault can reliably act.
Example 7: f. of 2 Point asymmetric short circuit fault, transition resistance 20 omega
On the basis of the calculation example 3, the fault point is changed to f 2 The point, the fault point transition resistance is 20 Ω, and the distortion rate of the characteristic harmonic current negative sequence component on both sides of the feeder MN after the fault is shown in fig. 17 (a) -17 (c).
As can be seen from fig. 17 (a) -17 (c), when a cycle after a single-phase ground fault, a two-phase ground fault and a two-phase short circuit fault occurs in the MN region of the feeder, the difference of the distortion rates at the two sides of the feeder is stabilized at 0.52, 0.34 and 0.15, and thus f is found to be 2 The difference value of distortion rates of two sides of a protected feeder line under the high-resistance asymmetric fault is far larger than an action threshold value, and both characteristic harmonic current differential protection can reliably act.
Example 8: f. of 2 Point-symmetric short-circuit fault with transition resistance of 0.1 omega
On the basis of the formula 4, the fault point is changed to f 2 The transition resistance of the fault point is 20 omega, and the distortion rate of the A-phase characteristic harmonic current on two sides of the feeder line MN after the fault is shown in FIG. 18.
As can be seen from FIG. 18, the difference of the characteristic harmonic current distortion of the A-phase on both sides of the feeder MN is about 3.5% after being stabilized, and is slightly lower than 4%, which is consistent with the analysis of the example 4, therefore, f 2 The characteristic harmonic current differential protection can still reliably act under the point high-resistance symmetrical fault.
Example two
The embodiment discloses an active injection type harmonic current differential protection system for a power distribution network containing an IIDG, which comprises:
a fault detection and harmonic injection control unit configured to: acquiring current information and voltage information at an outlet of the inversion distributed power supply in real time; judging whether a starting condition is met or not according to current information and voltage information at an outlet of the inversion distributed power supply, and starting an injection unit if the starting condition is met;
an injection unit configured to: injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter distributed power supply controller, and outputting characteristic harmonic currents by the inverter distributed power supply;
a differential protection unit configured to: and calculating the distortion rate difference value of the negative sequence component of the characteristic harmonic current on the two sides of the protected line and the distortion rate difference value of the A-phase characteristic harmonic current on the two sides of the protected line according to the characteristic harmonic current, judging whether the differential protection criterion is met, and if so, executing a protection action.
It should be noted here that the fault detection and harmonic injection control unit, the injection unit, and the differential protection unit correspond to the steps in the first embodiment, and the units and the corresponding steps implement the same examples and application scenarios, but are not limited to the disclosure in the first embodiment. It should be noted that the above-described elements as part of a system may be implemented in a computer system, such as a set of computer-executable instructions.
In the foregoing embodiments, the descriptions of the embodiments have different emphasis, and for parts that are not described in detail in a certain embodiment, reference may be made to related descriptions of other embodiments.
The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made to the present application by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims (10)
1. An active injection type harmonic current differential protection method for a power distribution network containing an IIDG is characterized by comprising the following steps:
s1, current information and voltage information at an outlet of an inversion distributed power supply are obtained in real time;
s2, judging whether the starting criterion is met or not according to the current information and the voltage information at the outlet of the inversion distributed power supply, and if so, continuing to execute the following steps; if not, returning to execute the step S1;
s3, starting an injection unit, injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter distributed power supply controller respectively, and outputting characteristic harmonic currents by the inverter distributed power supply;
and S4, calculating the distortion rate difference value of the negative sequence component of the characteristic harmonic current on the two sides of the protected line and the distortion rate difference value of the A-phase characteristic harmonic current on the two sides of the protected line according to the characteristic harmonic current, judging whether a differential protection criterion is met, and if so, executing a protection action.
2. The active injection harmonic current differential protection method for an IIDG-containing power distribution network of claim 1 wherein the start criteria comprise phase voltage start criteria and phase current start criteria, wherein the phase current start criteria are expressed as:
in the formula ,
for the number of sampling points corresponding to the current time, is greater than or equal to>
Number of samples taken for a cycle>
A number of samples taken in a cycle preceding the current time>
Is the sampling point number of the previous two cycles at the current moment, is counted and then is judged>
For the sampled value of the phase current at the present time, is->
For the sampled value of the phase current of the cycle preceding the present time, is->
For the sampled value of the phase current of the two cycles preceding the present moment, is->
A start coefficient and a value of 0.1 to 0.3, based on the measured values>
Is the rated current of the protected feeder.
3. The active injection type harmonic current differential protection method for the distribution network with the IIDG of claim 1, wherein the step S1 further comprises:
if the start-up criterion is met, an enable signal is sent to the injection unit to start the injection unit.
4. The active injection type harmonic current differential protection method for the distribution network with the IIDG of claim 1, wherein the characteristic harmonic current of the a phase is represented as:
wherein ,
is the amplitude of the characteristic harmonic current>
Is the phase angle of a characteristic harmonic signal>
Is the phase angle of power frequency>
Angular frequency for characteristic harmonic signals>
Is the angular frequency of the power frequency>
For the current time instant>
For the frequency of a characteristic harmonic signal injected into the d-axis, q-axis reference current, <' > or>
At power frequency>
Is the initial phase angle of the characteristic harmonic signal.
5. The active injection type harmonic current differential protection method for the distribution network containing the IIDG as claimed in claim 4, wherein the amplitude a of the characteristic harmonic current is expressed as
wherein ,IIBDG_d Is a d-axis reference current, I IIDG_q Is a q-axis reference current.
6. The active injection type harmonic current differential protection method for the distribution network containing the IIDG of claim 1, wherein the calculating the difference of the distortion rates of the characteristic harmonic currents on the two sides of the protected line is as follows:
respectively calculating negative sequence components in short-circuit currents on two sides of a protected line, wherein the negative sequence components are negative sequence components of characteristic harmonic currents;
respectively calculating the distortion rate of the negative sequence component of the characteristic harmonic current according to the negative sequence component of the characteristic harmonic current and the negative sequence component of the power frequency current;
calculating the absolute value of the difference in the negative sequence component distortion rates;
the calculation of the difference value of the A-phase characteristic harmonic current distortion at two sides of the protected line is as follows:
respectively calculating A-phase characteristic harmonic current distortion rates at two sides of the protected line according to the amplitude of the A-phase characteristic harmonic current and the amplitude of the A-phase power frequency current;
the absolute value of the difference in the a-phase characteristic harmonic current distortion rate is calculated.
7. The active injection type harmonic current differential protection method for the distribution network containing the IIDG of claim 6, wherein the judging whether the differential protection criterion is satisfied is:
judging whether the absolute value of the difference value of the negative sequence component distortion rate of the characteristic harmonic current in the short-circuit current on two sides of the protected line is greater than a first differential protection action threshold value or not;
judging whether the absolute value of the difference value of the A-phase characteristic harmonic current distortion rate in the short-circuit current on the two sides of the protected line is greater than a second differential protection action threshold value or not;
if the difference value is greater than the first differential protection action threshold value or greater than the second differential protection action threshold value, executing differential protection action and stopping the injection of the characteristic harmonic signal;
the first differential protection action threshold and the second differential protection action threshold are equal or unequal.
8. The active injection type harmonic current differential protection method for the power distribution network containing the IIDG of claim 1, wherein the calculation of the distortion rate of the negative sequence component of the characteristic harmonic current is represented as follows:
wherein h is the number of characteristic harmonics,
is the amplitude of the negative sequence component of the h-th characteristic harmonic current>
The amplitude of the negative sequence component of the power frequency current.
9. The active injection type harmonic current differential protection method for the distribution network containing the IIDG of claim 1, wherein the frequency of the characteristic harmonic current is not more than 10 times of power frequency and is an integral multiple of the power frequency.
10. An active injection type harmonic current differential protection system for a power distribution network containing an IIDG is characterized by comprising:
a fault detection and harmonic injection control unit configured to: acquiring current information and voltage information at an outlet of the inversion distributed power supply in real time; judging whether a starting condition is met or not according to current information and voltage information at an outlet of the inversion distributed power supply, and starting an injection unit if the starting condition is met;
an injection unit configured to: injecting characteristic harmonic signals into d-axis and q-axis reference currents of a current inner ring control module in the inverter distributed power supply controller, and outputting characteristic harmonic currents by the inverter distributed power supply;
a differential protection unit configured to: and calculating the distortion rate difference value of the negative sequence component of the characteristic harmonic current on the two sides of the protected line and the distortion rate difference value of the A-phase characteristic harmonic current on the two sides of the protected line according to the characteristic harmonic current, judging whether a differential protection criterion is met, and if so, executing a protection action.
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