CN112531767A - Neutral point grounding mode and single-phase grounding fault positioning method for medium-voltage island microgrid - Google Patents
Neutral point grounding mode and single-phase grounding fault positioning method for medium-voltage island microgrid Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
- G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/04—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for transformers
- H02H7/05—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for transformers for capacitive voltage transformers, e.g. against resonant conditions
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y04—INFORMATION OR COMMUNICATION TECHNOLOGIES HAVING AN IMPACT ON OTHER TECHNOLOGY AREAS
- Y04S—SYSTEMS INTEGRATING TECHNOLOGIES RELATED TO POWER NETWORK OPERATION, COMMUNICATION OR INFORMATION TECHNOLOGIES FOR IMPROVING THE ELECTRICAL POWER GENERATION, TRANSMISSION, DISTRIBUTION, MANAGEMENT OR USAGE, i.e. SMART GRIDS
- Y04S10/00—Systems supporting electrical power generation, transmission or distribution
- Y04S10/50—Systems or methods supporting the power network operation or management, involving a certain degree of interaction with the load-side end user applications
- Y04S10/52—Outage or fault management, e.g. fault detection or location
Abstract
The invention discloses a neutral point grounding mode and a single-phase grounding fault positioning method of a medium-voltage island microgrid, which comprises the following steps of: after the microgrid enters an island mode, a neutral point which is grounded through a voltage clamp low-resistance is put into the microgrid to provide a channel for releasing charges; after a fault occurs, introducing an auxiliary contact of a first breaker of the microgrid grid-connected into a controller, and when detecting that a normally open contact of the first breaker is disconnected, controlling a second breaker of the breaker to be closed by the controller, and putting a resonance elimination grounding transformer and a voltage clamping low resistance into the controller; establishing a three-sequence network by using a symmetric component method to obtain a composite sequence network; calculating the zero sequence current of the system according to a formula; the fault point is located between the detection point on a certain line where the zero sequence current is detected to be greater than the starting value and the detection point where the zero sequence current is detected to be less than the starting value. For the island operation microgrid system, a neutral point is constructed in a mode of grounding through a small resistor, ferromagnetic resonance can be effectively eliminated, and meanwhile, accurate single-phase grounding fault positioning can be realized by utilizing the distribution characteristics of the zero-sequence current of the system.
Description
Technical Field
The invention belongs to the technical field of power grid fault detection, and particularly relates to a neutral point grounding mode and a single-phase grounding fault positioning method for a medium-voltage island micro-grid.
Background
The microgrid is a small-sized distribution power system composed of a distributed power supply, a power load, a power distribution device, an energy storage device, a monitoring and protecting device and the like, is one of future development directions of a power grid, is very common when being connected to a 10-35 kV power grid, and is particularly important for determining a neutral point grounding mode of an island microgrid, a single-phase grounding fault positioning technology and the like. Usually, a microgrid is composed of a plurality of business entities, and each entity needs to independently measure voltage through an electromagnetic voltage transformer (PT), so that compared with a traditional high-voltage distribution network, the microgrid has the characteristic that a plurality of PTs run in parallel. The operation modes of the microgrid comprise grid-connected operation and island operation. Under the island operation mode, the microgrid loses a main network neutral point arc extinction coil or voltage clamping of a small resistor, and at the moment, the parallel operation PT is easy to generate ferromagnetic resonance, so that the selection and fault location of a neutral point grounding mode of the system are influenced.
Many experts and scholars research on a conventional neutral point grounding mode of a power distribution network, but design of a neutral point grounding mode of an island micro-grid is not considered. Many experts have conducted extensive studies on the mechanism and suppression measures of ferroresonance, but not much on the mechanism and suppression measures of multi-PT ferroresonance existing in island micro-grids.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provides a neutral point grounding mode and a single-phase grounding fault positioning method for a medium-voltage island microgrid.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a neutral point grounding mode and a single-phase grounding fault positioning method for a medium-voltage island microgrid comprise the following steps:
after the microgrid enters an island mode, a neutral point which is grounded through a voltage clamp low-resistance is quickly put into the microgrid to provide a channel for releasing charges;
introducing an auxiliary contact of a first breaker for grid connection of the microgrid into a controller, and when detecting that a normally open contact of the first breaker is disconnected, controlling a second breaker of the breaker to be closed by the controller, and putting a resonance elimination grounding transformer and a voltage clamping low resistance into the controller;
establishing a three-sequence network by using a symmetric component method to obtain a composite sequence network;
according to the formula
Calculating the zero sequence current of the system, wherein
Is a positive sequence network equivalent voltage source, z∑1Is a positive sequence equivalent impedance, z∑2Is a negative sequence equivalent impedance, Z∑0Is zero sequence equivalent impedance;
respectively three-phase current. Carrying out central differential derivation on the zero sequence current, and then taking an absolute value to obtain a sequence I0For sequence I0The first cycle data X in (1)0Making a judgment if X0(n)>0.5max{I0Taking the modulus value at the moment n as the pulse interference and setting the pulse interference to zero to obtain a first cycle sequence without interference, and taking I as0The modulus values below a threshold K, where K is 1.5max { X0}。
Setting the starting value of zero-sequence current as I0triggerAnd the fault point is located between the detection point on a certain line, which detects that the zero-sequence current is greater than the starting value, and the detection point, which detects that the zero-sequence current is less than the starting value.
For the island operation microgrid system, a neutral point is constructed in a mode of grounding through a small resistor, ferromagnetic resonance can be effectively eliminated, and meanwhile, accurate single-phase grounding fault positioning can be realized by utilizing the distribution characteristics of the zero-sequence current of the system. .
Drawings
Fig. 1 is a diagram of a microgrid structure provided in the practice of the present invention;
FIG. 2 is a schematic diagram of a three-phase ferroresonant circuit provided in the practice of the present invention;
FIG. 3 is a schematic diagram of a voltage-clamped low resistance control circuit in accordance with an embodiment of the present invention;
fig. 4 is a topological structure diagram of a one-week medium-voltage island micro-grid provided by the implementation of the present invention;
FIG. 5 is a schematic diagram of a composite orderlike network provided in accordance with an embodiment of the present invention;
FIG. 6 is a graph of system neutral voltage after disengagement from the main network in accordance with an embodiment of the present invention;
FIG. 7 is a graph of system neutral voltage after an input voltage clamp low resistance provided by an implementation of the present invention;
fig. 8 is a zero sequence current diagram of the detection point 2 provided in the embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and the detailed description.
As shown in fig. 1, a situation that a plurality of PTs (electromagnetic voltage transformers) are operated in parallel easily occurs in a microgrid in an island mode, when a main grid fails or power quality does not meet requirements, a common coupling Point (PCC) is disconnected, the microgrid is separated from the main grid and enters the island mode, and a neutral point of the main grid, which is grounded through a small resistor or an arc suppression coil, is lost.
The three-phase ferroresonant circuit is shown in FIG. 2, where eAeBeCIs a three-phase supply potential, LALBLCExcitation inductance of three-phase PT, RARBRCThe high side resistance of PT and the system capacitance to ground. For the purpose of analysis, it is assumed that the n parallel PT in-phase windings in the figure have equal inductance flux linkages and are all ψi(i＝A,B,C)，u0Is the power supply neutral point voltage. The following equations can be set according to kirchhoff's law:
the neutral point voltage, phase voltage and phase current under the resonance condition can be obtained through the differential equation set, but the analytic solution of the equation set cannot be obtained at present, and a problem set carries out a large amount of numerical analysis and calculation, and the result is shown in table 1.
TABLE 1 relationship of parallel PT number to neutral point voltage and resonant frequency
As can be seen from table 1, no resonance occurs with only one PT; as the number of parallel PTs increases and the equivalent L of the parallel multiple PTs becomes smaller, frequency doubling resonance occurs more easily.
The neutral point is grounded through a small resistor, so that resonance energy can be consumed, and the neutral point voltage can be clamped at a small value due to the small grounding resistor, so that ferromagnetic resonance is eliminated. A large amount of field practical operation experience shows that ferromagnetic resonance can not occur when a neutral point is grounded through a resistor of 10-20 omega, so that the paper adopts the following steps: after the microgrid enters an island mode, a neutral point which is grounded through a voltage clamp low-resistance is quickly put into use to provide a channel for releasing electric charges, and even if a plurality of PTs are connected in parallel in the system, resonance can still be effectively eliminated.
As shown in fig. 3, in the research, an auxiliary contact of a microgrid-connected circuit breaker QF1 is introduced into a controller, when the open contact of QF1 is detected to be open, the controller controls the circuit breaker QF2 to be closed, a resonance-eliminating grounding transformer and a voltage clamping low resistance are put into the circuit breaker, and the resonance-eliminating grounding transformer and the voltage clamping low resistance are quickly put into the circuit breaker, so that effective resonance elimination is guaranteed.
As shown in fig. 1, in the island microgrid topology, the system is composed of three inverter type DGs, DG1 is in a V/f control mode, DG2 and DG3 are in a PQ control mode, and when a single-phase ground short circuit occurs at a fault point 1, a three-sequence network is established by using a symmetric component method, so that a composite sequence network shown in fig. 4 is obtained.
According to
The zero sequence current of the system can be calculated. Wherein
Is a positive sequence network equivalent voltage source, z∑1Is a positive sequence equivalent impedance, z∑2Is a negative sequence equivalent impedance, z∑0Is the equivalent impedance of the zero sequence,
respectively three-phase current.
Because the wiring modes of the DG and the high-voltage side winding of the load transformer are generally angular connection, the PT angle is open connection, the excitation impedance is very large, and zero-sequence current only flows between a short-circuit point and the grounding transformer, and a loop is formed through the large ground.
And the composite sequence network is a distributed power supply under VF control, and the short-circuit model is a voltage source.
The short circuit model of the distributed power supply under PQ control in the composite sequence network is a current source and only has a positive sequence component.
Because the high-voltage side winding of the transformer of the distributed power supply and the load is generally not in star connection, the zero-sequence current only flows between a short-circuit point and a voltage clamping low resistor and forms a loop through the ground in the established composite sequence network.
Detecting the zero sequence currents of all the detection points, and obtaining the zero sequence currents of all the detection points by collecting the sum of the three-phase currents of the detection points and dividing the sum by 3, wherein the zero sequence currents are as follows:
carrying out central differential derivation on the zero sequence current, and then taking an absolute value to obtain a sequence I0And the central differential derivation processing process comprises the following steps:
h (n +1) and h (n-1) are values of the zero sequence current data at the time n +1 and the time n-1 respectively, and h' (n) is a value at the time n after derivation.
For sequence I0The first cycle data X in (1)0Making a judgment if X0(n)>0.5max{I0And (4) regarding the modulus value at the moment n as pulse interference and setting the pulse interference to zero to obtain a first cycle sequence for removing interference, wherein X is0(n) is X0The value of n at time instant.
Will I0The modulus values below the threshold K, where K is 1.5max { X }, are considered noise zeroed0}. Selecting effective interval from the result after zero setting, the internal model maximum value of the interval is effectiveAnd sampling a zero-sequence current value.
Setting the starting value of zero-sequence current as I0triggerSetting the detection point with the effective zero sequence current sampling value larger than the starting value as 1 and the detection point smaller than the starting value as 0, and according to the value taking conditions of all the detection points of the system, the fault point is positioned between two adjacent detection points with the value respectively being 1 and 0 on a certain line.
In order to verify the correctness of the scheme, an island microgrid shown in fig. 4 is built through MATLAB/Simulink simulation software, the microgrid consists of 3 distributed power supplies, 3 PTs are connected in parallel, a resonance elimination grounding transformer T and a voltage clamp low resistance R are additionally arranged to eliminate ferromagnetic resonance, R is 10 omega, I0trigger10A. When the circuit breaker connected with the main network is disconnected, the system neutral point voltage is as shown in fig. 6, as can be seen from fig. 6, the neutral point of the main network grounding is lost, and the frequency tripling ferromagnetic resonance occurs in the microgrid system in the "island mode".
As shown in fig. 7, when t is 0.2 seconds, the microgrid is disconnected from the main grid, and when t is 0.4 seconds, a voltage-clamped low resistor with a resistance of 10 Ω is applied, as can be seen from fig. 7, by adopting this resonance elimination measure, the neutral point voltage is quickly restored to a normal value, and the ferromagnetic resonance is effectively eliminated.
Setting the starting value of the zero-sequence current to be 10A, generating an A-phase short circuit at a fault point 1 in 2s, calculating the zero-sequence currents of all the detection points, and carrying out center difference derivation and noise zero setting processing on the zero-sequence currents to obtain that the zero-sequence current only at the detection point 2 is larger than 10A, as shown in FIG. 8.
Setting the detection point with the detected zero-sequence current larger than 10A as 1, setting the detection point with the detected zero-sequence current smaller than 10A as 0, setting the value of the detection point 2 as 1, and setting the values of other detection points as 0, wherein the value-taking conditions of all the detection points of the system are as shown in table 2:
TABLE 2 detection Point values
As can be seen from table 2, the short point is located between detection point 1 and detection point 2.
The above examples are merely for illustrative clarity and are not intended to limit the embodiments. Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art in light of the above teachings. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are within the scope of the invention.
Claims (8)
1. A neutral point grounding mode and a single-phase grounding fault positioning method for a medium-voltage island microgrid comprise the following steps:
after the microgrid enters an island mode, a neutral point which is grounded through a voltage clamp low-resistance is put into the microgrid to provide a channel for releasing charges;
after a fault occurs, introducing an auxiliary contact of a first breaker of the microgrid grid-connected into a controller, and when detecting that a normally open contact of the first breaker is disconnected, controlling a second breaker of the breaker to be closed by the controller, and putting a resonance elimination grounding transformer and a voltage clamping low resistance into the controller;
establishing a three-sequence network by using a symmetric component method to obtain a composite sequence network;
calculating zero sequence currents of all detection points;
setting the starting value of zero-sequence current as I0triggerAnd the fault point is located between the detection point on a certain line, which detects that the zero-sequence current is greater than the starting value, and the detection point, which detects that the zero-sequence current is less than the starting value.
2. The method of claim 1, wherein the composite distribution network is a VF-controlled distributed power source and the short-circuit model is a voltage source.
3. The method of claim 1, wherein the complex sequence net is a PQ-controlled distributed power source with a short-circuit model of current source and only positive sequence component.
4. The method according to claim 1, characterized in that zero sequence currents in the composite sequence net only flow between a short-circuit point and a voltage-clamped low resistance, forming a loop through earth.
5. The positioning method according to claim 1, wherein the zero sequence current of the computing system needs to detect the zero sequence currents of all the detection points, and specifically, the zero sequence current of each detection point is obtained by collecting the sum of the three phase currents of the detection points and dividing the sum by 3:
6. the positioning method according to claim 5, further comprising after obtaining zero sequence current, performing central differential derivation on the zero sequence current, and then taking an absolute value to obtain sequence I0And the central differential derivation processing process comprises the following steps:
h (n +1) and h (n-1) are values of the zero sequence current data at the moment n +1 and the moment n-1 respectively, and h' (n) is a value at the moment n after derivation; for sequence I0The first cycle data X in (1)0Making a judgment if X0(n)>0.5max{I0And (4) regarding the modulus value at the moment n as pulse interference and setting the pulse interference to zero to obtain a first cycle sequence for removing interference, wherein X is0(n) is X0The value at time n.
7. The positioning method according to claim 6, further comprising: will I0The modulus values below the threshold K, where K is 1.5max { X }, are considered noise zeroed0}。
8. The positioning method according to claim 7, further comprising: in the introduction I0And selecting an effective interval from the result after the modulus value lower than the threshold value K is regarded as the noise zero, wherein the internal model maximum value of the interval is an effective zero sequence current sampling value.
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