CN118399321A - Control method and system for coping with degradation of overcurrent protection sensitivity of power distribution network - Google Patents
Control method and system for coping with degradation of overcurrent protection sensitivity of power distribution network Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
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- H02H1/0092—Details of emergency protective circuit arrangements concerning the data processing means, e.g. expert systems, neural networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
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Abstract
The invention discloses a control method and a system for coping with degradation of overcurrent protection sensitivity of a power distribution network, wherein the method comprises the following steps: obtaining sensitivity degradation rate representation of the overcurrent relay on the corresponding main feed line according to a certain generalized three-phase distributed power grid; the electric faults cause voltage dip of the public coupling point, and reference currents of the inverter under different fault levels are obtained; and respectively obtaining reference currents according to the active reference power and the reactive reference power, and controlling the inverter output current through a pulse width modulation strategy according to the obtained reference currents of the inverter so as to maintain the fault ride-through capability of the distributed power supply based on the inverter. According to the method, the fault current analysis of the main circuit breaker is established, the influence of the problem of the protection dead zone is analyzed according to the worst case of the high-impedance fault at the tail end of the feeder line, and meanwhile, the total fault current is obtained, so that a basis is provided for providing a control strategy for relieving sensitivity degradation, the cost of upgrading equipment is reduced, and the safe operation of the active power distribution network is ensured.
Description
Technical Field
The invention relates to the technical field of protection of active power distribution networks, in particular to a control method and a system for coping with degradation of overcurrent protection sensitivity of a power distribution network.
Background
With the large-scale deployment of the distributed power supply, the power supply quality of the power grid is greatly improved, however, the selectivity and the response time of the overcurrent protection system are also negatively influenced, the protection misoperation is caused, and the reliability of the power grid is further reduced. The problem of degradation of the overcurrent protection sensitivity caused by the distributed power supply will significantly interfere with the stable operation of the protection system.
In the context of the access of a large number of distributed power generation devices, the problem of degradation of the sensitivity of over-current protection has attracted much attention, and many studies have been made to alleviate this problem. At present, most schemes for relieving sensitivity degradation only can roughly protect a power distribution network under partial conditions, and are difficult to accurately solve the problem of different fault protection blind areas. Therefore, the capacity should be determined in the protection of the allowable embedded distributed power supply (DG) to prevent the protection problem in terms of coordination, and in order to solve this problem, a mitigation strategy is proposed for synchronous DGs in an unbalanced power grid, which plays an important role in improving the domestic active power distribution network protection technology and the device market competitiveness.
The traditional active distribution network protection control strategy is to set the adaptive protection of a time dialing device (TDS) by searching for the optimal time scale of an overcurrent relay (OCR). The method is long in time consumption, and the combined distribution network of the relay and the fuse is not considered in actual conditions, so that not only can the efficiency be reduced, but also the equipment cost can be increased, and the safety of the power distribution network cannot be guaranteed. Therefore, how to properly deal with these problems becomes an important and difficult point of the power industry.
Disclosure of Invention
The invention aims to: in order to overcome the defects of the prior art, the invention provides a control method for coping with the degradation of the overcurrent protection sensitivity of the power distribution network, solves the problem of the degradation of the sensitivity of the current relay caused by the quantity of distributed power sources in the prior art, and also provides a control system for coping with the degradation of the overcurrent protection sensitivity of the power distribution network.
The technical scheme is as follows: according to a first aspect of the present invention, there is provided a control method for coping with degradation of overcurrent protection sensitivity of a power distribution network, the method comprising the steps of:
s1, obtaining sensitivity degradation rate representation of an overcurrent relay on a corresponding main feed line according to a generalized three-phase distributed power grid, wherein at least one distributed power supply based on an inverter exists in the generalized three-phase distributed power grid, and an electrical fault exists in the power distribution network;
s2, the electric faults cause voltage dip of the public coupling point, and the voltage drop value is used as a reference voltage, so that the reference current of the inverter under different fault levels is obtained;
s3, based on fault classification, respectively obtaining reference currents under an alpha beta coordinate system according to active reference power and reactive reference power, wherein the fault classification comprises symmetrical faults and asymmetrical faults;
And S4, controlling the inverter output current through a pulse width modulation strategy according to the reference current of the inverter obtained in the step S3 so as to maintain the fault ride-through capability of the distributed power supply based on the inverter.
Further, the method comprises the steps of:
In the step S2, the reference currents of the inverter under different fault levels are obtained specifically:
s21, judging whether the power grid is normally interfered or power grid fault according to a comparison value of the preset voltage and the reference voltage;
s22, if the power grid fault is normal interference, calculating the reference current of the inverter at the moment, otherwise, if the power grid fault is normal interference, judging the grade of the power grid fault at first;
s23, obtaining the reference current of the inverter according to the grade of the power grid fault.
Further, the method comprises the steps of:
the step S21 specifically includes:
V ref is the voltage drop value of the PCC in the generalized three-phase distributed power grid, V set is the preset voltage, and the voltage drop value is 0.88Pu;
The reference current of the inverter is expressed as:
Wherein P out is the output of the direct current side voltage regulator, I set is a preset current, a is a sensitivity constant which is used for evaluating the fault level according to historical experience, a is more than or equal to 2 and less than or equal to 6,k and is a constant, and the calculation formula is as follows:
further, the method comprises the steps of:
The step S3 specifically comprises the following steps:
S31, if the fault is classified as a symmetrical fault, calculating the reference current of the inverter through active power P ref and reactive reference power Q ref;
S32, if the faults are classified as asymmetric faults, positive sequence and negative sequence analysis are carried out on the output current of the inverter according to the instantaneous power values of the active power and the reactive power, and reference voltage and reference current are generated for the inverter; and updating the active power and the reactive power according to the generated reference voltage and reference current, thereby obtaining the reference current under the alpha beta coordinate system.
Further, the method comprises the steps of:
In the step S31, the reference current of the inverter is expressed as:
Where V α and V β are values of V ref converted to αβ coordinate system, P ref is active power of the inverter, Q ref is reactive power of the inverter, and the reactive power Q ref is expressed as:
Where I max represents the maximum output current of the inverter and V represents the voltage magnitude of the point of common coupling PCC in a generalized three-phase distributed power grid.
Further, the method comprises the steps of:
The step S32 specifically includes:
S321 based on positive and negative sequence components of instantaneous values of active and reactive power and estimating reference voltage at asymmetric fault And a reference current
S322 updates the reference power by using the reference voltage and the reference current obtained in the step S321, and calculates a reference value of the injection current required in the fault according to the reference power.
Further, the method comprises the steps of:
The updating of the reference power with the reference voltage and the reference current is expressed as:
Wherein, As a positive sequence component of the active power,As a negative sequence component of the active power,Is the positive sequence component of the reactive power,As a negative sequence component of the reactive power,As a positive sequence component of the reference voltage in the alpha direction,As a negative sequence component of the reference voltage in the alpha direction,As a positive sequence component of the reference voltage in the beta direction,As a negative sequence component of the reference voltage in the beta direction,For a positive sequence component of the reference current in the alpha direction,As a negative sequence component of the reference current in the alpha direction,For a positive sequence component of the reference current in the beta direction,Is the negative sequence component of the reference current in the beta direction;
Calculating a reference value of current to be injected in the fault according to the reference power, wherein the reference value specifically comprises:
Wherein i α(p)、iβ(p) is the active component of the reference current in the alpha beta coordinate system, and i α(q)、iβ(q) is the reactive component of the reference current in the alpha beta coordinate system;
Thus, the current reference value is expressed as:
in another aspect, the present invention further provides a control system for coping with degradation of overcurrent protection sensitivity of a power distribution network, where the system includes:
The sensitivity degradation rate representation module is used for obtaining sensitivity degradation rate representation of the overcurrent relay on the corresponding main feed line according to a certain generalized three-phase distributed power grid, wherein at least one distributed power supply based on an inverter exists in the generalized three-phase distributed power grid, and an electrical fault exists in the power distribution network;
The first reference current generation module is used for taking the voltage drop value as a reference voltage according to voltage dip of the public coupling point caused by the electric fault, so as to obtain reference currents of the inverter under different fault levels;
the second reference current generation module is used for respectively obtaining reference currents under an alpha beta coordinate system according to active reference power and reactive reference power on the basis of fault classification, wherein the fault classification comprises symmetrical faults and asymmetrical faults;
and the adjusting module is used for controlling the output current of the inverter through a pulse width modulation strategy according to the reference current so as to maintain the fault ride-through capability of the distributed power supply based on the inverter.
Further, the method comprises the steps of:
In the first reference current generating module, the obtaining the reference current of the inverter under different fault levels specifically includes:
judging whether the power grid is normally interfered or power grid fault according to a comparison value of the preset voltage and the reference voltage;
If the power grid fault is normal interference, calculating the reference current of the inverter at the moment, otherwise, judging the grade of the power grid fault if the power grid fault is abnormal;
and obtaining the reference current of the inverter according to the grade of the power grid fault.
Further, the method comprises the steps of:
the second reference current generation module specifically includes:
If the fault is classified as a symmetrical fault, calculating the reference current of the inverter through active power P ref and reactive reference power Q ref;
If the fault is classified as an asymmetric fault, positive sequence and negative sequence analysis are carried out on the output current of the inverter according to the instantaneous power values of the active power and the reactive power, and reference voltage and reference current are generated for the inverter; and updating the active power and the reactive power according to the generated reference voltage and reference current, thereby obtaining the reference current under the alpha beta coordinate system.
The beneficial effects are that: compared with the prior art, the invention has the following advantages: the invention first establishes a sensitivity degradation SD model. On the basis of combining the existing SD model, a main breaker fault current analysis type is established, then the influence of a protection blind area problem is analyzed according to the worst case of a high-impedance fault at the tail end of a feeder line, and meanwhile, the total fault current is obtained, so that a basis is provided for further providing a control strategy for relieving sensitivity degradation, the protection blind area problem is solved, the cost of upgrading equipment is reduced, and the safe operation of an active power distribution network is ensured. The strategy provided by the invention can reasonably limit fault current, avoid damage of electrical equipment, relieve the problem of sensitivity degradation, distinguish normal interference from power grid faults, has stronger anti-interference capability and improves the power supply reliability.
Drawings
FIG. 1 is a diagram of an exemplary generalized distributed network according to an embodiment of the present invention;
Fig. 2 is a flowchart of a control method for coping with degradation of overcurrent protection sensitivity of a power distribution network according to an embodiment of the invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
In order to propose a control strategy for alleviating the problem of sensitivity degradation, the invention needs to establish a Sensitivity Degradation (SD) model, in particular:
When distributed power sources (DG) in a distribution network are more concentrated, the reduced sensitivity of the over-current relays (OCR) on the main feeder can cause serious protection hazards. The substantial reduction in the OCR-sensed fault level can greatly delay the protection operation time, ultimately resulting in the loss of backup protection for the downstream power distribution system (PD). In order to build the SD model, a generalized Distributed Network (DN) is proposed, and as shown in fig. 1, a three-phase fault occurs at a fault point "F", and the fault impedance is Z f. The fault current (I f,CB) through the main breaker can be expressed as follows (1.1):
Wherein E G and Z G represent the source voltage and complex impedance of the upstream circuit, respectively. Z X represents the line impedance between bus bars A-B, and Z Y represents the line impedance between bus bars B-C. I f,DG denotes a fault current from the integrated DG unit.
The failure point (Z Y+Zf≈0)＜＜ZG+ZX, (1.1) near DG can be expressed as follows (1.2):
in the worst case of High Impedance Failure (HIF) at the feeder end, Z Y+Zf＞＞ZG+ZX, (1.1) can be revised as follows:
Equation (1.3) shows that the SD value of the over-current PD will be high for the far-end HIF. The problem of blindness of protection is even more serious as DG penetration levels increase.
In addition, considering the situation that DG is disposed at the end of the distribution feeder, the total fault current I F can be estimated by the theory of the equivalent circuit of davin as follows:
wherein Z DG is DG impedance.
I f,CB can be calculated using the following formula:
Let Z c＝ZG+ZX+ZY, and define Substitution (1.4) to obtain
For fault current I f,CB near DG fault point (where Z Y is negligible) it can be expressed as:
in general, it is possible to use The DG capacity is indicated as being,Representing the comprehensive power generation capacity of the power grid, and further obtaining:
from the above equation, γ can be estimated using MVA DG、MVAC.
I f,CB can be calculated from formulas (1.6) and (1.7), and can further be derived from the following formulas:
Prior to DG access, I f,CB may be expressed as:
The Sensitivity Degradation Rate (SDR) can be calculated from the formula (1.9) and the formula (1.10):
Wherein,
Therefore, the equation (1.11) shows that when the more DG are connected in the power distribution network, the lower the sensitivity of the overcurrent protection device is, the longer the delay of triggering protection is, which seriously jeopardizes the operation safety of the power network, and the control strategy of DG must be optimized for the problem.
The control strategy for optimizing DG specifically comprises:
Inverter-based distributed power supplies (IBDG) account for a relatively large percentage of power distribution networks, and thus overcurrent protection sensitivity degradation mitigation strategies have been designed for IBDG. When the fault current amplitude is large, the fault current of the IBDG can be limited by turning off the current transformer. However, this also reduces system reliability and limits the fault ride-through capability of IBDG. Accordingly, the fault current may be limited by varying the reference current signal I c,f.
The proposed strategy is summarized as follows: an electrical fault may cause a Point of Common Coupling (PCC) to experience a voltage dip, which may be used as a reference voltage V ref for identifying fault conditions and estimating fault levels. This voltage drop can also be used as a detection signal to detect and limit the fault current of the IBDG. This will improve the fault ride through capability of the IBDG during grid faults. Another advantage of this strategy is the ability to distinguish between a slight disturbance and a grid fault based on the reference voltage V ref. The strategy compares V ref with a preset voltage V set to distinguish normal disturbances from grid faults:
Typically, V set can be selected to be 0.88pu. This scheme would continue to observe V ref to evaluate the reference signal indicator of IBDG controlled output current under fault conditions. After any fault occurs, the voltage dip at the reference point will cause a large change in the reference signal, thereby limiting the output current of the inverter. The reference current may be defined as follows:
under normal interference conditions, I c,f can be calculated from the reference power P out; in the fault condition, a is a sensitivity constant, and is used for evaluating the fault level, and a is generally 2-6. k may be obtained by assuming I c,f to be constant:
From (2.3), the proposed strategy will limit the output current of the inverter according to the fault level. For low impedance faults near DG, the output current of the inverter will decrease to a very low value due to the small value of I c,f. For ease of calculation, V ref and inverter output current I m are converted from the abc coordinate system to the αβ coordinate system based on a Clark transformation. I c,f is added in the formulas (2.6) and (2.9), I c,f is added in the reference current formula, which is equivalent to reducing the reference current, and finally reducing the output current
For a symmetrical fault, the reference current can be calculated from the active and reactive reference power (P ref、Qref):
The reference current of the inverter is expressed as:
Where V α and V β are values of V ref converted to αβ coordinate system, P ref is active power of the inverter, Q ref is reactive power of the inverter, and the reactive power Q ref is expressed as:
Where I max represents the maximum output current of the inverter and V represents the voltage magnitude of the point of common coupling PCC in a generalized three-phase distributed power grid.
In the case of an asymmetrical fault, the inverter output current can be analyzed for positive and negative sequence from instantaneous values of active and reactive power. The instantaneous power value can be further decomposed into positive and negative sequence components to estimate the reference voltage at the time of an asymmetric faultAnd a reference currentAn auxiliary controller can be designed for the negative sequence loop, the reference signal is adjusted according to the fault level, and a new reference voltage is generated for the inverter by adding the adjusted negative sequence voltage to the positive sequence voltage. Then, the reference power is updated with the reference voltage according to equation (2.8).
Wherein,As a positive sequence component of the active power,As a negative sequence component of the active power,Is the positive sequence component of the reactive power,As a negative sequence component of the reactive power,As a positive sequence component of the reference voltage in the alpha direction,As a negative sequence component of the reference voltage in the alpha direction,As a positive sequence component of the reference voltage in the beta direction,As a negative sequence component of the reference voltage in the beta direction,For a positive sequence component of the reference current in the alpha direction,As a negative sequence component of the reference current in the alpha direction,For a positive sequence component of the reference current in the beta direction,Is the negative sequence component of the reference current in the beta direction;
Calculating a reference value of current to be injected in the fault according to the reference power, wherein the reference value specifically comprises:
Wherein i α(p)、iβ(p) is the active component of the reference current in the alpha beta coordinate system, and i α(q)、iβ(q) is the reactive component of the reference current in the alpha beta coordinate system;
Thus, the current reference value is expressed as:
the updated reference power will be used to calculate the reference value of the current that needs to be injected at the time of the fault, see equations (2.9) and (2.10).
And according to the judged fault type, the output current of the inverter can be controlled through a pulse width modulation strategy after the corresponding reference current is obtained so as to maintain the fault ride-through capability of the IBDG.
In summary, the provided strategy can reasonably limit fault current, avoid damage of electrical equipment, relieve the problem of sensitivity degradation, distinguish normal interference from power grid faults, have stronger anti-interference capability and improve power supply reliability.
According to the control strategy mentioned above, the present invention provides a control method for coping with degradation of overcurrent protection sensitivity of a power distribution network, the method comprising the steps of:
s1, obtaining sensitivity degradation rate representation of an overcurrent relay on a corresponding main feed line according to a generalized three-phase distributed power grid, wherein at least one distributed power supply based on an inverter exists in the generalized three-phase distributed power grid, and an electrical fault exists in the power distribution network;
And S2, the electric fault causes voltage dip of the public coupling point, and the voltage drop value is used as a reference voltage, so that the reference current of the inverter under different fault levels is obtained.
The reference current of the inverter under different fault levels is obtained, and the reference current is specifically expressed as:
S21, judging whether the power grid is normally interfered or failed according to a comparison value of the preset voltage and the reference voltage.
Wherein V ref is the reference voltage, V set is a preset voltage, and the value of the voltage is 0.88Pu;
The reference current of the inverter is expressed as:
Wherein P out is the output of the direct current side voltage regulator, I set is a preset current, a is a sensitivity constant which is used for evaluating the fault level according to historical experience, a is more than or equal to 2 and less than or equal to 6,k and the calculation formula is as follows:
s22, if the power grid fault is normal interference, calculating the reference current of the inverter at the moment, otherwise, if the power grid fault is normal interference, judging the grade of the power grid fault at first;
s23, obtaining the reference current of the inverter according to the grade of the power grid fault.
S3, based on fault classification, respectively obtaining reference currents under an alpha beta coordinate system according to active reference power and reactive reference power, wherein the fault classification comprises symmetrical faults and asymmetrical faults;
The step S3 specifically comprises the following steps:
if the fault is classified as a symmetrical fault in S31, the reference current of the inverter is calculated by the active power P ref and the reactive reference powers Q ref and I c,f.
The reference current of the inverter is expressed as:
Where V α and V β are values of V ref converted to αβ coordinate system, P ref is active power of the inverter, Q ref is reactive power of the inverter, and the reactive power Q ref is expressed as:
Where I max represents the maximum output current of the inverter and V represents the voltage magnitude of the point of common coupling PCC in a generalized three-phase distributed power grid.
S32, if the faults are classified as asymmetric faults, positive sequence and negative sequence analysis are carried out on the output current of the inverter according to the instantaneous power values of the active power and the reactive power, and a new reference voltage is generated for the inverter; and updating the active power and the reactive power according to the generated new reference voltage, thereby obtaining the reference current under the alpha beta coordinate system.
The step S32 specifically includes:
S321 based on positive and negative sequence components of instantaneous values of active and reactive power and estimating reference voltage at asymmetric fault And a reference current
S322, updating reference power by using the reference voltage, and calculating a reference value of current to be injected in the fault according to the reference power;
Updating the reference power with the new reference voltage is expressed as:
Wherein, As a positive sequence component of the active power,As a negative sequence component of the active power,Is the positive sequence component of the reactive power,As a negative sequence component of the reactive power,As a positive sequence component of the reference voltage in the alpha direction,As a negative sequence component of the reference voltage in the alpha direction,As a positive sequence component of the reference voltage in the beta direction,As a negative sequence component of the reference voltage in the beta direction,For a positive sequence component of the reference current in the alpha direction,As a negative sequence component of the reference current in the alpha direction,For a positive sequence component of the reference current in the beta direction,Is the negative sequence component of the reference current in the beta direction;
Calculating a reference value of current to be injected in the fault according to the reference power, wherein the reference value specifically comprises:
Wherein i α(p)、iβ(p) is the active component of the reference current in the alpha beta coordinate system, and i α(q)、iβ(q) is the reactive component of the reference current in the alpha beta coordinate system;
Thus, the current reference value is expressed as:
And S4, controlling the output current of the inverter through a pulse width modulation strategy according to the reference current of the inverter obtained in the step S3 so as to maintain the fault ride-through capability of the distributed power supply based on the inverter. That is, firstly, the fault type is judged, and the inverter output current is controlled through a pulse width modulation strategy according to the corresponding reference current.
On the other hand, the invention also provides a control system for coping with the degradation of the overcurrent protection sensitivity of the power distribution network, which comprises:
The sensitivity degradation rate representation module is used for obtaining sensitivity degradation rate representation of the overcurrent relay on the corresponding main feed line according to a certain generalized three-phase distributed power grid, wherein at least one distributed power supply based on an inverter exists in the generalized three-phase distributed power grid, and an electrical fault exists in the power distribution network;
The first reference current generation module is used for taking the voltage drop value as a reference voltage according to voltage dip of the public coupling point caused by the electric fault, so as to obtain reference currents of the inverter under different fault levels;
the second reference current generation module is used for respectively obtaining reference currents under an alpha beta coordinate system according to active reference power and reactive reference power on the basis of fault classification, wherein the fault classification comprises symmetrical faults and asymmetrical faults;
And the adjusting module is used for controlling the inverter output current through a pulse width modulation strategy according to the reference current so as to maintain the fault ride-through capability of the distributed power supply based on the inverter.
In the first reference current generating module, the obtaining the reference current of the inverter under different fault levels specifically includes:
judging whether the power grid is normally interfered or power grid fault according to a comparison value of the preset voltage and the reference voltage;
If the power grid fault is normal interference, calculating the reference current of the inverter at the moment, otherwise, judging the grade of the power grid fault if the power grid fault is abnormal;
and obtaining the reference current of the inverter according to the grade of the power grid fault.
The second reference current generation module specifically includes:
If the fault is classified as a symmetrical fault, calculating the reference current of the inverter through active power P ref and reactive reference power Q ref;
If the fault is classified as an asymmetric fault, positive sequence and negative sequence analysis are carried out on the output current of the inverter according to the instantaneous power values of the active power and the reactive power, and reference voltage and reference current are generated for the inverter; and updating the active power and the reactive power according to the generated reference voltage and reference current, thereby obtaining the reference current under the alpha beta coordinate system.
Other technical features of the control system for coping with the degradation of the power distribution network overcurrent protection sensitivity are the same as those of the control method for coping with the degradation of the power distribution network overcurrent protection sensitivity, and are not repeated here.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. It is therefore intended that the following claims be interpreted as including the preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments of the present invention without departing from the spirit or scope of the embodiments of the invention. Thus, if such modifications and variations of the embodiments of the present invention fall within the scope of the claims and the equivalents thereof, the present invention is also intended to include such modifications and variations.
Claims (10)
1. A control method for coping with degradation of overcurrent protection sensitivity of a power distribution network, the method comprising the steps of:
s1, obtaining sensitivity degradation rate representation of an overcurrent relay on a corresponding main feed line according to a generalized three-phase distributed power grid, wherein at least one distributed power supply based on an inverter exists in the generalized three-phase distributed power grid, and an electrical fault exists in the power distribution network;
s2, the electric faults cause voltage dip of the public coupling point, and the voltage drop value is used as a reference voltage, so that the reference current of the inverter under different fault levels is obtained;
s3, based on fault classification, respectively obtaining reference currents under an alpha beta coordinate system according to active reference power and reactive reference power, wherein the fault classification comprises symmetrical faults and asymmetrical faults;
And S4, controlling the inverter output current through a pulse width modulation strategy according to the reference current of the inverter obtained in the step S3 so as to maintain the fault ride-through capability of the distributed power supply based on the inverter.
2. The method for controlling degradation of sensitivity to overcurrent protection of a power distribution network according to claim 1, wherein in step S2, the reference currents of the inverters at different fault levels are obtained specifically:
s21, judging whether the power grid is normally interfered or power grid fault according to a comparison value of the preset voltage and the reference voltage;
s22, if the power grid fault is normal interference, calculating the reference current of the inverter at the moment, otherwise, if the power grid fault is normal interference, judging the grade of the power grid fault at first;
s23, obtaining the reference current of the inverter according to the grade of the power grid fault.
3. The method according to claim 2, wherein the step S21 is specifically:
V ref is the voltage drop value of the PCC in the generalized three-phase distributed power grid, V set is the preset voltage, and the voltage drop value is 0.88Pu;
The reference current of the inverter is expressed as:
Wherein P out is the output of the direct current side voltage regulator, I set is a preset current, a is a sensitivity constant which is used for evaluating the fault level according to historical experience, a is more than or equal to 2 and less than or equal to 6,k and is a constant, and the calculation formula is as follows:
4. the method for controlling degradation of sensitivity to overcurrent protection of a power distribution network according to claim 1, wherein the step S3 specifically includes the steps of:
S31, if the fault is classified as a symmetrical fault, calculating the reference current of the inverter through active power P ref and reactive reference power Q ref;
S32, if the faults are classified as asymmetric faults, positive sequence and negative sequence analysis are carried out on the output current of the inverter according to the instantaneous power values of the active power and the reactive power, and reference voltage and reference current are generated for the inverter; and updating the active power and the reactive power according to the generated reference voltage and reference current, thereby obtaining the reference current under the alpha beta coordinate system.
5. The method according to claim 4, wherein in step S31, the reference current of the inverter is expressed as:
Where V α and V β are values of V ref converted to αβ coordinate system, P ref is active power of the inverter, Q ref is reactive power of the inverter, and the reactive power Q ref is expressed as:
Where I max represents the maximum output current of the inverter and V represents the voltage magnitude of the point of common coupling PCC in a generalized three-phase distributed power grid.
6. The method for controlling degradation of sensitivity to overcurrent protection of a power distribution network according to claim 4, wherein the step S32 specifically includes:
S321 based on positive and negative sequence components of instantaneous values of active and reactive power and estimating reference voltage at asymmetric fault And a reference current
S322 updates the reference power by using the reference voltage and the reference current obtained in the step S321, and calculates a reference value of the injection current required in the fault according to the reference power.
7. The method for controlling degradation of sensitivity to over-current protection of a power distribution network according to claim 6, wherein updating the reference power with the reference voltage and the reference current is expressed as:
Wherein, As a positive sequence component of the active power,As a negative sequence component of the active power,Is the positive sequence component of the reactive power,As a negative sequence component of the reactive power,As a positive sequence component of the reference voltage in the alpha direction,As a negative sequence component of the reference voltage in the alpha direction,As a positive sequence component of the reference voltage in the beta direction,As a negative sequence component of the reference voltage in the beta direction,For a positive sequence component of the reference current in the alpha direction,As a negative sequence component of the reference current in the alpha direction,For a positive sequence component of the reference current in the beta direction,Is the negative sequence component of the reference current in the beta direction;
Calculating a reference value of current to be injected in the fault according to the reference power, wherein the reference value specifically comprises:
Wherein i α(p)、iβ(p) is the active component of the reference current in the alpha beta coordinate system, and i α(q)、iβ(q) is the reactive component of the reference current in the alpha beta coordinate system;
Thus, the current reference value is expressed as:
8. a control system for handling degradation of overcurrent protection sensitivity of a power distribution network, the system comprising:
The sensitivity degradation rate representation module is used for obtaining sensitivity degradation rate representation of the overcurrent relay on the corresponding main feed line according to a certain generalized three-phase distributed power grid, wherein at least one distributed power supply based on an inverter exists in the generalized three-phase distributed power grid, and an electrical fault exists in the power distribution network;
The first reference current generation module is used for taking the voltage drop value as a reference voltage according to voltage dip of the public coupling point caused by the electric fault, so as to obtain reference currents of the inverter under different fault levels;
the second reference current generation module is used for respectively obtaining reference currents under an alpha beta coordinate system according to active reference power and reactive reference power on the basis of fault classification, wherein the fault classification comprises symmetrical faults and asymmetrical faults;
and the adjusting module is used for controlling the output current of the inverter through a pulse width modulation strategy according to the reference current so as to maintain the fault ride-through capability of the distributed power supply based on the inverter.
9. The control system for coping with degradation of overcurrent protection sensitivity of a power distribution network according to claim 8, wherein the obtaining the reference currents of the inverters at different fault levels in the first reference current generating module specifically includes:
judging whether the power grid is normally interfered or power grid fault according to a comparison value of the preset voltage and the reference voltage;
If the power grid fault is normal interference, calculating the reference current of the inverter at the moment, otherwise, judging the grade of the power grid fault if the power grid fault is abnormal;
and obtaining the reference current of the inverter according to the grade of the power grid fault.
10. The control system for coping with degradation of sensitivity to overcurrent protection of a power distribution network according to claim 8, wherein the second reference current generating module specifically comprises:
If the fault is classified as a symmetrical fault, calculating the reference current of the inverter through active power P ref and reactive reference power Q ref;
If the fault is classified as an asymmetric fault, positive sequence and negative sequence analysis are carried out on the output current of the inverter according to the instantaneous power values of the active power and the reactive power, and reference voltage and reference current are generated for the inverter; and updating the active power and the reactive power according to the generated reference voltage and reference current, thereby obtaining the reference current under the alpha beta coordinate system.
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