Power distribution network three-section type overcurrent protection starting auxiliary method based on reactive current output characteristics of photovoltaic power supply

Abstract

The invention discloses a photovoltaic power reactive current output characteristic-based power distribution network three-section type overcurrent protection starting auxiliary method, which comprises the following steps of: according to the characteristic that photovoltaic outputs reactive power to a power grid under low voltage ride through, determining the change characteristic of a reactive current effective value output by the photovoltaic when a fault occurs; and monitoring the effective value of the photovoltaic reactive current during the fault period, judging whether the effective value of the photovoltaic reactive current during the fault period is within the action threshold, identifying whether the fault position is in the overcurrent protection area, and determining whether to trigger the protection action logic or not according to the judgment result. According to the invention, the problem of three-section type over-current protection maloperation caused by increasing assisting and drawing-out effects of the photovoltaic power supply is effectively eliminated, the action reliability of the protection device is remarkably improved, and the protection device is not influenced by the access of the photovoltaic power supply.

Description

Technical Field

[0001] The present invention relates to the field of power systems and automation technologies thereof, and in particular to a three-stage overcurrent protection start-up auxiliary method for a distribution network based on reactive current output characteristics of a photovoltaic power source. Background Art

[0002] With the continuous growth of the grid-connected capacity of distributed generation (DG), the proportion of distributed photovoltaic (PV) has gradually increased. Its impact on the safe and stable operation of the distribution network is becoming increasingly significant. The traditional distribution network was originally a single-power radiation structure with unidirectional current flow; the access of distributed photovoltaic power sources has transformed the network into a complex multi-power system with bidirectional current flow characteristics, resulting in increased uncertainty in the direction and magnitude of the current during a fault, making it difficult for the original overcurrent protection scheme to adapt. Summary of the invention

[0003] In order to solve the technical problems existing in the above-mentioned prior art, the present invention proposes a three-stage overcurrent protection starting auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply. Based on the reactive output characteristics of photovoltaic under low voltage ride-through state, by introducing the output of reactive current during fault period into the starting criterion of traditional overcurrent protection, a set of protection starting auxiliary strategies that can effectively clear faults are established, thereby improving the reliability of distribution network protection.

[0004] To achieve the above object, the present invention provides a three-stage overcurrent protection startup auxiliary method for a distribution network based on the reactive current output characteristics of a photovoltaic power source, comprising:

[0005] According to the characteristics of reactive power output from photovoltaic to the grid under low voltage ride-through, the change characteristics of the effective value of reactive current output from photovoltaic during fault are determined;

[0006] Monitor the effective value of photovoltaic reactive current during the fault period and determine whether to trigger the protection action logic based on the judgment basis.

[0007] Preferably, determining the change characteristics of the effective value of the reactive current output of the photovoltaic device during the fault includes:

[0008] Construct a photovoltaic equivalent model with low voltage ride-through capability;

[0009] The output characteristics of the photovoltaic during low voltage ride-through are obtained by using the photovoltaic equivalent model with low voltage ride-through capability;

[0010] Based on the output characteristics of the photovoltaic power generation during low voltage ride-through, the voltage drop characteristics of the grid connection point at different fault locations are analyzed, and the effective value of the reactive current output in the low voltage ride-through mode is derived.

[0011] Preferably, constructing a photovoltaic equivalent model with low voltage ride-through capability includes:

[0012] Considering the reactive power support requirements of the photovoltaic power station, the output of active power P and reactive power Q of the photovoltaic inverter is obtained according to the definition of instantaneous power, and the d-axis and q-axis components of the inverter output current are obtained by deduction;

[0013] The active and reactive power output by the photovoltaic inverter is controlled by the d-axis and q-axis components of the inverter output current, thereby obtaining the dynamic reactive current injected by the photovoltaic inverter into the power grid, that is, the photovoltaic equivalent model with low voltage ride-through capability.

[0014] Preferably, the photovoltaic equivalent model with low voltage ride-through capability is:

[0015]

[0016] Where U T I is the voltage of the photovoltaic power grid connection point; N is the rated current of the photovoltaic power source; U N is the system rated voltage, I' qref The dynamic reactive current injected into the grid by the PV inverter.

[0017] Preferably, the output characteristics during the photovoltaic low voltage ride-through period are:

[0018] When U T / U N >When the preset value is 1, the photovoltaic does not transmit reactive current to the outside, but only outputs active power;

[0019] When the preset value 2 < U T / U N When the voltage drops below the preset value, the photovoltaic system enters the low voltage ride-through mode and provides a certain amount of dynamic reactive power support to the grid according to the voltage drop at the grid connection point. At this time, reactive current is fed outward.

[0020] When U T / U N When ≤ preset value 2, the photovoltaic system outputs a certain amount of reactive current, and the photovoltaic system acts as a constant current source.

[0021] Preferably, analyzing the voltage drop characteristics of the grid connection point at different fault locations based on the output characteristics during the photovoltaic low voltage ride-through period includes:

[0022] When the fault occurs at the head end of the line, the reactive current output by the photovoltaic power plant is:

[0023]

[0024] When the fault occurs at the end of the line, the reactive current output by the photovoltaic power plant is:

[0025]

[0026] In the formula, is the reactive current of the photovoltaic output when the line head fails, is the reactive current of the photovoltaic output when the line ends are faulty, I N is the rated current of the photovoltaic power source, U N is the system rated voltage, P is the capacity of the photovoltaic power station, β1 is the impedance angle of the downstream line of the photovoltaic power station, and Z R is the equivalent impedance downstream of the photovoltaic power station, Z BC is the BC line impedance, β BC is the BC line impedance angle, Z AC is the AC line impedance, β AC is the AC line impedance angle.

[0027] Preferably, the action threshold interval is obtained according to the reactive current of the photovoltaic output when the fault occurs at the head end of the line and the reactive current of the photovoltaic output when the fault occurs at the end of the line, specifically:

[0028]

[0029] In the formula, I' qref The dynamic reactive current injected into the grid by the PV inverter.

[0030] Preferably, the judgment basis is:

[0031] Condition 1: the effective value of the monitored reactive current is within the action threshold range;

[0032] Condition 2: The fault current flowing through the line protection exceeds the setting threshold given by the traditional overcurrent protection method.

[0033] Preferably, if both condition one and condition two are met, the action logic is met and the protection action is performed; otherwise, the action logic is not met and the protection does not take place.

[0034] Compared with the prior art, the present invention has the following advantages and technical effects:

[0035] (1) Based on the reactive output characteristics of photovoltaic power under low voltage ride-through state, the present invention introduces the output of reactive current during fault period into the start-up criterion of traditional overcurrent protection, and establishes a set of protection start-up auxiliary strategies that can effectively clear faults, thereby improving the reliability of distribution network protection and being applicable to protection start-up auxiliary methods for different fault conditions.

[0036] (2) The present invention enhances the accuracy of the protection action, and can still maintain the reliability of the action even if there is a certain penetration rate of distributed photovoltaics in the distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0038] Figure 1 This is a flow chart of a three-stage overcurrent protection startup auxiliary method for a distribution network based on the reactive current output characteristics of a photovoltaic power source according to an embodiment of the present invention;

[0039] Figure 2 A typical distribution network system topology diagram of a distribution network three-stage overcurrent protection start-up auxiliary method based on the reactive current output characteristics of a photovoltaic power source according to an embodiment of the present invention;

[0040] Figure 3 This is a typical three-phase fault equivalent Thevenin circuit diagram of a distribution network system according to an embodiment of the present invention, which is a three-stage overcurrent protection start-up auxiliary method for a distribution network based on the reactive current output characteristics of a photovoltaic power source. DETAILED DESCRIPTION

[0041] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0042] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0043] The present invention proposes a three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply, such as Figure 1 ,include:

[0044] According to the characteristics of reactive power output from photovoltaic to the grid under low voltage ride-through, the change characteristics of the effective value of reactive current output from photovoltaic during fault are determined;

[0045] Monitor the effective value of photovoltaic reactive current during the fault period and determine whether to trigger the protection action logic based on the judgment basis.

[0046] This embodiment is based on the reactive output characteristics of photovoltaic low voltage ride-through state. By introducing the output of reactive current during the fault period into the start-up criterion of traditional overcurrent protection, a set of protection start-up auxiliary strategies that can effectively clear the fault are established, thereby improving the reliability of distribution network protection.

[0047] Furthermore, the change characteristics of the effective value of the reactive current output of the photovoltaic power plant during a fault are determined, including:

[0048] Construct a photovoltaic equivalent model with low voltage ride-through capability;

[0049] The output characteristics of the photovoltaic during low voltage ride-through are obtained by using the photovoltaic equivalent model with low voltage ride-through capability;

[0050] Based on the output characteristics of the photovoltaic power generation during low voltage ride-through, the voltage drop characteristics of the grid connection point at different fault locations are analyzed, and the effective value of the reactive current output in the low voltage ride-through mode is derived.

[0051] Specifically, constructing a photovoltaic equivalent model with low voltage ride-through capability includes:

[0052] When a system failure causes the grid connection point voltage to drop, considering the requirements for reactive power support of the photovoltaic power station, the photovoltaic power source should transmit dynamic reactive power to the grid. According to the definition of instantaneous power, the output of active power P and reactive power Q of the photovoltaic inverter can be obtained as follows:

[0053]

[0054] In the formula, u d ,u q are the d-axis and q-axis components of the inverter output voltage, respectively. d 、i q They are the d-axis and q-axis components of the inverter output current respectively.

[0055] From the above formula, we can deduce that i d 、i q The expression is:

[0056]

[0057] Therefore, by controlling i d 、i q The active and reactive power output by the photovoltaic inverter can be controlled.

[0058] In case of a fault, the dynamic reactive current I' injected by the PV inverter into the grid qref for:

[0059]

[0060] Where U T I is the voltage of the photovoltaic power grid connection point; N is the rated current of the photovoltaic power source; U N is the rated voltage of the system.

[0061] Furthermore, the output characteristics of the photovoltaic low voltage ride-through period are:

[0062] When U T / U N>When the preset value is 1, the photovoltaic does not transmit reactive current to the outside, but only outputs active power;

[0063] When the preset value 2 < U T / U N When the voltage drops below the preset value, the photovoltaic system enters the low voltage ride-through mode and provides a certain amount of dynamic reactive power support to the grid according to the voltage drop at the grid connection point. At this time, reactive current is fed outward.

[0064] When U T / U N When the value is less than or equal to the preset value 2, the photovoltaic system outputs a certain amount of reactive current. At this time, it is regarded as a constant current source and is disconnected from the grid after a period of time.

[0065] In this embodiment, the first preset value is 0.9, and the second preset value is 0.2.

[0066] Furthermore, by analyzing the voltage drop characteristics of the grid connection point under different fault locations, the effective value of reactive current output in the low voltage ride-through mode is derived.

[0067] When the photovoltaic power plant is in the low voltage ride-through state, Figure 1 Taking the typical distribution network topology structure with three-phase short circuit as an example, the Thevenin equivalent circuit diagram is obtained, and the grid connection point voltage U is derived. T for:

[0068]

[0069] Where α is the ratio of the distance between the fault point and the PV power station to the total length of the line AC, P is the capacity of the PV power station, β1 is the impedance angle of the downstream line of the PV power station, β2 is the impedance angle of the fault line, and Z is R is the equivalent impedance downstream of the PV power station.

[0070] Substituting equation (4) into equation (3) we can deduce that the voltage at the grid connection point is The photovoltaic power station outputs reactive reference current I' in the state qref , specifically:

[0071]

[0072] Furthermore, based on the output characteristics of photovoltaic low voltage ride-through, the voltage drop characteristics of the grid connection point under different fault locations are analyzed, including:

[0073] When the fault occurs at the head end of the line, the reactive current output by the photovoltaic power plant is:

[0074]

[0075] When the fault occurs at the end of the line, the reactive current output by the photovoltaic power plant is:

[0076]

[0077] In the formula, is the reactive current of the photovoltaic output when the line head fails, is the reactive current of the photovoltaic output when the line ends are faulty, I N is the rated current of the photovoltaic power source, U N is the system rated voltage, P is the capacity of the photovoltaic power station, β1 is the impedance angle of the downstream line of the photovoltaic power station, and Z R is the equivalent impedance downstream of the photovoltaic power station, Z BC is the BC line impedance, β BC is the BC line impedance angle, Z AC is the AC line impedance, β AC is the AC line impedance angle.

[0078] Furthermore, the action threshold interval is obtained according to the reactive current of the photovoltaic output when the fault occurs at the head end of the line and the reactive current of the photovoltaic output when the fault occurs at the end of the line, which is specifically:

[0079]

[0080] In the formula, I' qref The dynamic reactive current injected into the grid by the PV inverter.

[0081] Specifically, the judgment is based on:

[0082] Condition 1: The effective value of the monitored reactive current is within the action threshold range;

[0083] Condition 2: The fault current flowing through the line protection exceeds the setting threshold given by the traditional overcurrent protection method.

[0084] If both condition one and condition two are met, it complies with the action logic and the protection action is taken; otherwise, it does not comply with the action logic and the protection does not take action.

[0085] This embodiment studies the reactive current output characteristics of photovoltaic power sources during low voltage crossing, and derives a three-stage overcurrent protection start-up auxiliary method for the distribution network based on the reactive current output characteristics of photovoltaic power sources. By constructing a photovoltaic low voltage crossing output model to obtain its reactive current output characteristics, and analyzing its output characteristics at different fault locations, an expression of the functional relationship between the reactive current output and the fault location under the low voltage crossing state is obtained. By constructing an auxiliary start-up criterion for overcurrent protection, the problem of false operation of the three-stage overcurrent protection caused by the auxiliary increase and external draw of photovoltaic power sources is effectively eliminated, which significantly improves the reliability of the protection device action and is not affected by the access of photovoltaic power sources.

[0086] In order to more clearly express the technical solution of the present invention, the following specific embodiments are provided to introduce the solution:

[0087] In the PSCAD simulation system, Figure 2 The typical distribution network structure of photovoltaic access is shown in Figure 2 to verify the effectiveness of the proposed method. The Thevenin equivalent circuit of the system after the fault occurs is shown in Figure 2 Figure 3 As shown. Three fault locations, namely the end of line AB, the end of line BC and the end of line AD, are selected for simulation verification. The protection action results are shown in Table 1. Table 2 shows the protection action of three-phase short circuit at different fault locations. It can be seen from Tables 1 and 2 that the protection start-up auxiliary criterion proposed in this embodiment has certain reliability in the typical distribution network structure of PSCAD.

[0088] Table 1

[0089]

[0090] Table 2

[0091]

[0092] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply, characterized in that: include: According to the characteristics of reactive power output from photovoltaic to the grid under low voltage ride-through, the change characteristics of the effective value of reactive current output from photovoltaic during fault are determined; Monitor the effective value of photovoltaic reactive current during the fault period and determine whether to trigger the protection action logic based on the judgment basis.

2. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 1 is characterized in that: Determine the change characteristics of the effective value of the reactive current output of the photovoltaic power plant during the fault, including: Construct a photovoltaic equivalent model with low voltage ride-through capability; The output characteristics of the photovoltaic during low voltage ride-through are obtained by using the photovoltaic equivalent model with low voltage ride-through capability; Based on the output characteristics of the photovoltaic power generation during low voltage ride-through, the voltage drop characteristics of the grid connection point at different fault locations are analyzed, and the effective value of the reactive current output in the low voltage ride-through mode is derived.

3. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 2 is characterized in that: Building a photovoltaic equivalent model with low voltage ride-through capability includes: Considering the reactive power support requirements of the photovoltaic power station, the output of active power P and reactive power Q of the photovoltaic inverter is obtained according to the definition of instantaneous power, and the d-axis and q-axis components of the inverter output current are obtained by deduction; The active and reactive power output by the photovoltaic inverter is controlled by the d-axis and q-axis components of the inverter output current, thereby obtaining the dynamic reactive current injected by the photovoltaic inverter into the power grid, that is, the photovoltaic equivalent model with low voltage ride-through capability.

4. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 3 is characterized in that: The photovoltaic equivalent model with low voltage ride-through capability is: Where U T I is the voltage of the photovoltaic power grid connection point; N is the rated current of the photovoltaic power source; U N is the system rated voltage, I ’ qref The dynamic reactive current injected into the grid by the PV inverter.

5. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 4 is characterized in that: The output characteristics during the photovoltaic low voltage ride-through period are: When U T / U N >When the preset value is 1, the photovoltaic does not transmit reactive current to the outside, but only outputs active power; When the preset value 2 < U T / U N When the voltage drops below the preset value, the photovoltaic system enters the low voltage ride-through mode and provides a certain amount of dynamic reactive power support to the grid according to the voltage drop at the grid connection point. At this time, reactive current is fed outward. When U T / U N When ≤ preset value 2, the photovoltaic system outputs a certain amount of reactive current, and the photovoltaic system acts as a constant current source.

6. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 2 is characterized in that: Based on the output characteristics of the photovoltaic low voltage ride-through period, the voltage drop characteristics of the grid connection point under different fault locations are analyzed, including: When the fault occurs at the head end of the line, the reactive current output by the photovoltaic power plant is: When the fault occurs at the end of the line, the reactive current output by the photovoltaic power plant is: In the formula, is the reactive current of the photovoltaic output when the line head fails, is the reactive current of the photovoltaic output when the line ends are faulty, I N is the rated current of the photovoltaic power source, U N is the system rated voltage, P is the capacity of the photovoltaic power station, β1 is the impedance angle of the downstream line of the photovoltaic power station, and Z R is the equivalent impedance downstream of the photovoltaic power station, Z BC is the BC line impedance, β BC is the BC line impedance angle, Z AC is the AC line impedance, β AC is the AC line impedance angle.

7. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 6 is characterized in that: The action threshold interval is obtained based on the reactive current output by the photovoltaic power plant when the fault occurs at the head end of the line and the reactive current output by the photovoltaic power plant when the fault occurs at the end of the line. Specifically, it is: In the formula, I ’ qref The dynamic reactive current injected into the grid by the PV inverter.

8. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 7 is characterized in that: The judgment is based on: Condition 1: the effective value of the monitored reactive current is within the action threshold range; Condition 2: The fault current flowing through the line protection exceeds the setting threshold given by the traditional overcurrent protection method.

9. The three-stage overcurrent protection startup auxiliary method for distribution network based on the reactive current output characteristics of photovoltaic power supply according to claim 8 is characterized in that: If both condition one and condition two are met, it complies with the action logic and the protection action is taken; otherwise, it does not comply with the action logic and the protection does not take action.

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