CN115730440B - Method for judging direct-current transmission harmonic current through current - Google Patents
Method for judging direct-current transmission harmonic current through current Download PDFInfo
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Abstract
The invention provides a direct-current transmission harmonic current flowing current judging method, which relates to the technical field of on-load tap-changers and comprises the steps of establishing a harmonic switching-off experimental model; calculating the current change rate of the on-load tap-changer based on experimental data of the harmonic switching-off experimental model, and generating a first database and a second database based on the current change rate and the value of the control angle alpha; calculating a current change rate margin value; and judging whether the DC transmission harmonic current of the on-load tap-changer is over-current or not. The current change rate margin value is calculated, the current change rate margin value is subtracted by the maximum current change rate to obtain a current change rate monitoring value, the current change rate monitoring value is used as the on-load tap-changer current change rate warning line, and whether the on-load tap-changer direct-current transmission harmonic current is over-current or not is judged by detecting the real-time current change rate and comparing the real-time current change rate with the current change rate monitoring value.
Description
Technical Field
The invention relates to the technical field of on-load tap-changer, in particular to a method for judging the flowing current of a direct-current transmission harmonic current.
Background
The converter transformer is a key device in a direct current transmission system, and realizes the electrical isolation of an alternating current system and a direct current system. The load current that the converter transformer on-load tap-changer needs to switch is a non-sinusoidal waveform with a large current rate of change (di/dt) compared to an ac power transformer on-load tap-changer. di/dt is a design parameter whose rate of change is much higher than that of an alternating current of 50Hz or 60 Hz. In an actual high-voltage direct-current transmission system, the total harmonic quantity of current flowing through a converter transformer and an on-load tap changer may not exceed a specified value, but the frequency range is very wide, and the influence on the recovery voltage of the on-load tap changer is very large. Especially in case of light loads of converter transformers, normal or accident operation may cause oscillation processes or resonance phenomena in the system. Therefore, the method has extremely important significance for monitoring and analyzing the overcurrent in the power system, and provides reliable and accurate reference for the safe operation of the power grid.
At present, a secondary current of a current transformer is generally used for driving a relay, or a method of rectifying the secondary current of the current transformer and comparing the rectified secondary current with a preset current setting value is adopted, or voltage signals formed by converting the secondary current are subjected to multi-point sampling in a cycle, effective values of the voltage signals are obtained by applying a Fourier algorithm and then are compared with the current setting value to confirm whether faults occur, harmonic components in the overcurrent have higher proportion of the total current, electric arcs are easy to occur, contact vibration and contact material electric abrasion caused by electric arc ablation when the on-load tap-changer is opened are caused by the electric arcs, the opening capacity of the on-load tap-changer is reduced, and an electric arc reburning phenomenon is also often caused after the electric arcs are extinguished when the on-load tap-changer is opened, so that the materials of contacts of the on-load tap-changer are further electric abraded, the on-load tap-changer is more easily broken down, and even accidents are caused.
Disclosure of Invention
The invention solves the technical problems that: the electric arc can cause contact vibration and electric abrasion of contact materials caused by electric arc ablation when the on-load tap-changer is opened, so that the opening capacity of the on-load tap-changer is reduced, and the phenomenon of arc reburning can also occur frequently after the electric arc is extinguished when the on-load tap-changer is opened, so that the materials of the contacts of the on-load tap-changer are further electrically abraded, the on-load tap-changer is more easily broken down, and even accidents are caused.
In order to solve the technical problems, the invention provides the following technical scheme: a method for judging the flowing current of a direct-current transmission harmonic current comprises the following steps:
establishing a harmonic break-off experimental model;
calculating the current change rate of the on-load tap-changer based on experimental data of the harmonic switching-off experimental model, and generating a first database and a second database based on the current change rate and the value of the control angle alpha;
calculating a current change rate margin value;
and judging whether the DC transmission harmonic current of the on-load tap-changer is over-current or not.
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: the computational expression of di/dt of the on-load tap-changer is as follows:
I d ＝I
Ur＝RI
wherein di/dt represents the current change rate, I d The main contact arc current is represented by Ur, the recovery voltage is represented by R, the on-load tap-changer resistance is represented by I, the rated current of the on-load tap-changer is represented by I, and the control angle is represented by α.
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: the recovery voltage rise on the main on-off contact of the on-load tap-changer is equal to the voltage drop generated at two ends of the transition resistor by current, when the recovery voltage is higher than the recovery rate of an insulating medium, the arc can reburns, the maximum recovery voltage is calculated, and the calculation expression is as follows:
maximum recovery voltage=r Transition ×(di/dt)；
Wherein R is Transition And the resistance value of the transition resistor is represented, and when the recovery voltage on the main on-off contact of the on-load tap-changer is lower than the maximum recovery voltage, the arc cannot be reburned.
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: calculating the current value of each control angle within the range of 0-90 DEG of the control angle alpha, and calculating the current change rate di/dt of the control angle alpha;
and (3) the calculated result of di/dt is in one-to-one correspondence with the control angle alpha, so as to obtain a first database.
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: a protection resistor is connected in series at one side of the on-load tap-changer, and the resistance value of the protection resistor is changed to simulate the resistance values of the on-load tap-changer in different states, and the maximum recovery voltage is calculated in a double-resistance mode at the moment, wherein the calculation expression is as follows;
Ur γ ＝IR protection of
Maximum recovery voltage Dual resistor ＝(di/dt γ +I(R-R Protection of ))/R Transition
Wherein di/dt γ Indicating the current change rate in the dual resistance mode, ur γ The recovery voltage in the double-resistance mode is represented, R represents the resistance of the on-load tap-changer, I represents the rated current of the on-load tap-changer, and alpha representsIndicating control angle, maximum recovery voltage Dual resistor Represents the maximum recovery voltage in the double-resistance mode, R Protection of The resistance value of the protection resistor is represented, the resistance value of the protection resistor corresponds to the on-off capability adjustment of the main on-off contact of the on-load tap changer, and as the on-off capability of the main on-off contact of the on-load tap changer is reduced along with the reduction of arc discharge, the visual representation is that the on-off capability is more sensitive to di/dt, the resistance of the on-load tap changer is gradually reduced, the resistance values of the historical on-load tap changers with different degrees of arc discharge are measured, and the resistance value R of the historical on-load tap changer is obtained History of The relational expression is as follows:
R-R history of ＝R Protection of 。
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: under the dual resistance mode, calculating the current value of each control angle within the range of 0-90 degrees of the control angle alpha;
and calculate the current change rate di/dt respectively γ ；
Will di/dt γ And (3) the calculation results of the control angles alpha are in one-to-one correspondence to obtain a second database.
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: grading the damage degree of a main on-off contact of the historical on-load tap-changer, wherein the grading comprises slight damage, moderate damage, severe damage and severe damage;
di/dt in a second database γ Subtracting di/dt from di/dt in the second database to obtain di/dt Δ ；
Will di/dt Δ The value is used as a current change rate margin value, and the current change rate margin value is classified according to the magnitude and the damage degree.
As a preferable scheme of the direct-current transmission harmonic current flowing current judging method of the invention, the method comprises the following steps: grading the damage degree of the on-off contact of the on-load tap-changer on the change current;
detecting and calculating the current change rate of the on-load tap-changer in real time during operation, and calculating the maximum current change rate under the maximum recovery voltage;
subtracting the current change rate margin value from the maximum current change rate to obtain a current change rate monitoring value;
comparing the value of the real-time current change rate with the monitored value of the current change rate;
and if the value of the real-time current change rate is larger than or equal to the monitoring value of the current change rate, triggering overcurrent early warning.
The invention has the beneficial effects that: the current change rate margin value is calculated, the current change rate margin value is subtracted by the maximum current change rate to obtain a current change rate monitoring value, the current change rate monitoring value is used as the on-load tap-changer current change rate warning line, and whether the on-load tap-changer DC power transmission harmonic current is over-current or not is judged by detecting the real-time current change rate and comparing the real-time current change rate with the current change rate monitoring value, so that arc re-ignition of the on-load tap-changer is avoided, and the on-load tap-changer is protected and the on-off capability is ensured. And the on-load tap-changer on-off contact damage degree can be used for adaptively adjusting the monitoring value of the current change rate of the on-load tap-changer on-off contact, controlling the on-load tap-changer on-off capability is finer, fully controlling the use state of the on-load tap-changer is facilitated, the service life of the on-load tap-changer is prolonged, and accidents are reduced.
Drawings
Fig. 1 is a flowchart of a method for judging a direct-current transmission harmonic current flowing through a current according to an embodiment of the present invention;
fig. 2 is a diagram of a voltage-regulating winding of a converter transformer in a method for judging a direct-current transmission harmonic current flowing through a current according to an embodiment of the present invention.
Fig. 3 is a schematic structural diagram of a pulsating rectifier bridge in a method for judging a direct-current transmission harmonic current flowing through a current according to an embodiment of the present invention.
Fig. 4 is a schematic diagram of a pulsating rectifier bridge current waveform in a method for determining a harmonic current flowing through a dc power transmission according to an embodiment of the present invention.
Fig. 5 is a waveform diagram of a recovery voltage on an on-load tap changer switch in a method for judging a direct-current transmission harmonic current flowing through a current according to an embodiment of the present invention; wherein, a graph represents the recovery voltage on the main on-off contact, and b graph represents the recovery voltage on the transition contact.
Fig. 6 is a schematic diagram of a recovery voltage waveform of a main contact of an on-load tap changer in a method for judging a direct-current transmission harmonic current flowing through a current according to an embodiment of the present invention; the graph a is the waveform of the recovery voltage of the converter transformer main contact connected with YNd, and the graph b is the waveform of the recovery voltage of the converter transformer main contact connected with YNy.
Detailed Description
In order that the above-recited objects, features and advantages of the present invention will become more readily apparent, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings.
The operation of the on-load tap-changer in the converter transformer has the following characteristics: 1. the pressure regulating range is large; 2. the action is frequent; 3. the switching-on and switching-off change frequency is high; 4. the direct current system is complex in harmonics. The above characteristics put higher demands on the on-load tap-changer of the converter transformer.
S1: and establishing a harmonic break experimental model.
The main factors affecting the arc breaking capability of an on-load tap changer contact are the breaking current through the contact and the recovery voltage developed across the contact break. Whether or not the arc is reburned when the on-load tap-changer is switched depends on the medium recovery and voltage recovery speeds. After the on-load tap-changer is determined in the type selection mode, the recovery speed of the contact medium is fixed, and only the recovery speed or the recovery voltage amplitude can be reduced to meet the requirement of arc extinction.
In general, an on-load tap changer connected to a neutral point is subjected to voltage adjustment by a network-side tap winding of a converter transformer, and thus the on-load tap changer is subjected to an on-load test as an experimental model. The winding arrangement of the converter transformer is shown in fig. 2.
When the on-load tap-changer is switched, after the main on-off contact is disconnected, load current still passes through the main on-off loop in the form of an electric arc until the electric arc is extinguished when the current waveform crosses zero, and the current is disconnected and transferred to the transition loop. The root-mean-square value and the rate of change of the recovery voltage occurring between the open contacts determine the open conditions of the contacts.
In a common power transformer, both current and voltage are pure sine waves, so that the recovery voltage is also sine waves, and the square root value and the change rate of the recovery voltage have a fixed relation, and only the square root value of the recovery voltage needs to be known.
The converter transformer converts the ac system voltage to the three-phase voltage required by the converter bridge. The 12-pulse converter comprises two three-phase bridge devices (6-pulse bridge) whose ac side has to be connected to two three-phase systems phase-shifted by 30 ° from each other, requiring two sets of converter transformer, one set being Y/Y connection and the other set being Y/D connection. The structure of the 12-pulse converter bridge is shown in fig. 3, and the current waveform is shown in fig. 4.
As can be seen from fig. 4, the winding current experienced by the converter transformer and its on-load tap changer in the hvdc system is not sinusoidal, but rather resembles square waves, which have a large influence on the switching performance of the on-load tap changer switch. In particular, in the Y/delta connection method, the di/dt of the square wave current at the zero crossing of the current is greater than the steepness of the positive line current of the same effective value, and the change-over switch has enough breaking capacity to be broken.
In the resistive change-over switch of the flag cycle method or the multi-resistance cycle method, the recovery voltage at the break of the main on-off contact is equivalent to the voltage drop generated by the passing current on the transition resistance, and as a result, the recovery voltage is also square wave. When the switch operates in the spike-cycling method, the recovery voltage is the vector sum of the voltage drop and the stage voltage. Fig. 5 shows the principle waveforms of the recovery voltage on the main on-off contact port, one generated by sinusoidal current and the other by current in delta-tap converter transformer (the change-over switch is a flag-loop method).
In the subsequent switching operation, the transition contact must cut off the current at the recovery voltage. The recovery voltage on the transition contact is the vector sum of the voltage drop across the transition resistor and the stage voltage (flag cycle and multi-resistance cycle operation for the switch). Fig. 5 is a schematic diagram of such a recovery voltage.
In converter transformers, the load current is not a pure sine wave and the peak value and the rate of change are not in a fixed relationship. If the recovery voltage change rate is too fast, and exceeds the recovery speed of the insulation strength between the contacts, the fracture is broken down by the recovery voltage, and reburning occurs.
S2: and calculating the current change rate of the on-load tap-changer based on experimental data of the harmonic switching experimental model, and generating a first database and a second database based on the current change rate and the value of the control angle alpha.
The waveform of the recovery voltage of the on-load tap changer contacts determines the arc extinguishing capability of the fracture. Since the tapped winding load current of the converter transformer is not sinusoidal, but resembles a square wave, the recovery voltage at the main contact port of the diverter switch corresponds to the voltage drop across the transition resistance by the current, and thus the recovery voltage is also a square wave. The switching capacity of the on-load tap changer of the converter transformer is influenced in a decisive manner, not by the change of the current value itself, but by the rate of change di/dt when the network-side current flows through zero. Therefore, the recovery voltage generated by the square wave load current of the converter transformer must be verified and the steepness (i.e. di/dt) of the passing current at zero crossing must be known.
The main factors affecting di/dt are short circuit impedance, coupling group and firing angle of the converter transformer. The short circuit impedance of each converter transformer of the same converter station is basically the same, and the network side of each converter transformer adopts YNd or YNy connection, so di/dt is generally a function of the trigger angle. Only the di/dt value corresponding to the rated firing angle is usually provided, but the value at different firing angles is also important for the selection and design of the on-load tap-changer. The trigger angle affects both the peak value and the di/dt value of the current, and therefore must be considered comprehensively to use the most severe pair of data for the on-load tap-changer as a basis for selecting and designing the on-load tap-changer.
When the main on-off contact is opened, the recovery voltage between the fractures is equal to the voltage drop of the load current on the transition resistor. The waveforms of the recovery voltages of the main contacts of the on-load tap changers of the YNd and YNy converter transformer are shown in fig. 6.
For the converter transformer with YNd connection mode, the net side current I approximates to two step waves, the recovery voltage also approximates to two step waves, and the current is zero crossingIs steep and the recovery voltage is steepest in the form of a current, which is the most severe case for the main on-off contacts.
For the YNy connection type converter transformer, the net side current is approximately a square wave, di/dt at zero crossing of the square wave is steep, but the current does not immediately rise in the opposite direction, but remains zero.
The switching condition of the main on-off hoe of the on-load tap changer of the converter transformer in the YNy connection mode is generally lighter than that of the on-load tap changer in the YNd connection mode. The time from zero crossing after current commutation to the beginning of rising of recovered voltage is related to the magnitude of the commutation angle, and is indirectly influenced by the control angle.
The computational expression of di/dt of the on-load tap-changer is as follows:
I d ＝I
Ur＝RI
wherein di/dt represents the current change rate, I d The main contact arc current is represented by Ur, the recovery voltage is represented by R, the on-load tap-changer resistance is represented by I, the rated current of the on-load tap-changer is represented by I, and the control angle is represented by α.
The recovery voltage rise on the main on-off contact of the on-load tap-changer is equal to the voltage drop generated at two ends of the transition resistor by current, when the recovery voltage is higher than the recovery rate of an insulating medium, the arc can reburns, the maximum recovery voltage is calculated, and the calculation expression is as follows:
maximum recovery voltage=r Transition ×(di/dt)；
Wherein R is Transition And the resistance value of the transition resistor is represented, and when the recovery voltage on the main on-off contact of the on-load tap-changer is lower than the maximum recovery voltage, the arc cannot be reburned.
Calculating the current value of each control angle within the range of 0-90 DEG of the control angle alpha, and calculating the current change rate di/dt of the control angle alpha;
and (3) the calculated result of di/dt is in one-to-one correspondence with the control angle alpha, so as to obtain a first database.
An arc can be generated in the on-load tap-switching process, and the arc is extinguished at the current zero crossing point. When the arc is extinguished, the dielectric strength begins to recover, and the recovery speed of the dielectric strength must be greater than that of the recovery voltage.
Because of the specificity of the operation condition, the recovery voltage of the converter transformer rises quickly when the switching arc passes through the zero point, if the recovery voltage between the switching contacts rises too quickly and exceeds the recovery speed of the insulation strength between the switching contacts, the fracture is broken down again, and the switching failure is caused.
Since di/dt is determined by the system, if the recovery voltage rises too fast, it is only possible to reduce the transition resistance R Transition To reduce the recovery voltage Ur.
A protection resistor is connected in series at one side of the on-load tap-changer, and the resistance value of the protection resistor is changed to simulate the resistance values of the on-load tap-changer in different states, and the maximum recovery voltage is calculated in a double-resistance mode at the moment, wherein the calculation expression is as follows;
Ur γ ＝IR protection of
Maximum recovery voltage Dual resistor ＝(di/dt γ +I(R-R Protection of ))/R Transition
Wherein di/dt γ Indicating the current change rate in the dual resistance mode, ur γ The recovery voltage in the double-resistance mode is represented by R, the on-load tap-changer resistor is represented by I, the rated current of the on-load tap-changer is represented by I, the control angle is represented by alpha, and the maximum recovery voltage is represented by alpha Dual resistor Represents the maximum recovery voltage in the double-resistance mode, R Protection of The resistance value of the protection resistor is represented and is adjusted corresponding to the on-off capability of the main on-off contact of the on-load tap-changer, and the on-off capability of the main on-off contact of the on-load tap-changer is discharged along with the arcThe visual representation is that the resistance of the load tap switch is gradually reduced when the resistance is sensitive to di/dt, and the resistance value R of the load tap switch is obtained by measuring the resistance values of the load tap switches with different degrees of arc discharge History of The relational expression is as follows:
R-R history of ＝R Protection of 。
Under the dual resistance mode, calculating the current value of each control angle within the range of 0-90 degrees of the control angle alpha;
and calculate the current change rate di/dt respectively γ ；
Will di/dt γ And (3) the calculation results of the control angles alpha are in one-to-one correspondence to obtain a second database.
In the double-resistance switching mode, for the converter transformer of the YNd connection mode, the network side current I approximates to two step waves, and the loop current Ic is caused by the step voltage of the alternating current sine wave, so the loop current is sine wave. There is generally a phase difference between the fundamental wave of the net side current and the ac voltage, the net side current enters the commutation region, the corresponding di/dt is maximum, but the rate of change of the stage voltage is not the maximum. For a YNy connection type converter transformer, the net side current approximates a square wave. The net side current square wave is zero, the circulation is not zero yet, and the total current therefore does not cross zero. When the loop flows through zero, the square wave of the current at the net side is still zero, and the transition resistance R is not present Transition The voltage drop is caused thereby that the recovery voltage is only a component of the stage voltage.
S3: a current change rate margin value is calculated.
For an on-load tap-changer, the insulation recovery speed of the on-load tap-changer is fixed within the contact switching capability, and if the insulation recovery speed cannot meet the requirement, the recovery speed of the Ur can only be reduced. If the network side current di/dt of the converter transformer where the on-load tap-changer is located is particularly large, the transition resistance R is suitably reduced Transition To reduce du/dt. Transition resistance R Transition The degree of reduction is required to realize the opening and closing of the contacts only when the recovery voltage waveform among the breaks of the on-load tap-changer is within the insulation recovery curve of the on-load tap-changer, or the resistance is reduced after the on-load tap-changer is broken down, and the on-load tap-changer needs to be suitableLocally reducing the transition resistance R Transition To reduce du/dt so that the recovery voltage waveform between the on-load tap-changer discontinuities is within its insulation recovery curve to effect opening of the contacts.
Grading the damage degree of a main on-off contact of the historical on-load tap-changer, wherein the grading comprises slight damage, moderate damage, severe damage and severe damage;
di/dt in a second database γ Subtracting di/dt from di/dt in the second database to obtain di/dt Δ ；
Will di/dt Δ The value is used as a current change rate margin value, and the current change rate margin value is classified according to the magnitude and the damage degree.
On-load tap-changer with double-resistance switching mode, reducing transition resistance R Transition The voltage rising rate of the main contact can be reduced, but the transition contact has double effects: on the one hand, the voltage rising rate on the transition contact is along with the transition resistance R Transition Decrease with decreasing, on the other hand, with transition resistance R Transition The total current on and off the transition contact increases due to the increase in circulating current.
For a specific HVDC project, since di/dt is determined by the system, only the transition resistance R can be reduced Transition The value to reduce the voltage rise rate. Therefore, under the condition of the same capacity, the transition resistance of the on-load tap-changer of the converter transformer is higher than the transition resistance R of the on-load tap-changer of the common power transformer Transition The value is lower. But transition resistance R Transition The value of the contact cannot deviate from the theoretical optimal value too much, the switching tasks of the main on-off contact and the transition contact are required to be balanced, the average ablation of the two contacts is ensured to be equivalent as much as possible, and the switching sequence is not influenced.
S4: and judging whether the DC transmission harmonic current of the on-load tap-changer is over-current or not.
Grading the damage degree of the on-off contact of the on-load tap-changer on the change current;
detecting and calculating the current change rate of the on-load tap-changer in real time during operation, and calculating the maximum current change rate under the maximum recovery voltage;
subtracting the current change rate margin value from the maximum current change rate to obtain a current change rate monitoring value;
comparing the value of the real-time current change rate with the monitored value of the current change rate;
and if the value of the real-time current change rate is larger than or equal to the monitoring value of the current change rate, triggering overcurrent early warning.
The on-load tap-changer on-off contacts are different in damage degree, the maximum current change rate which can be born is different, the current change rate margin value is subtracted by the maximum current change rate, the current change rate monitoring value is obtained, the current change rate monitoring value is used as the on-load tap-changer current change rate warning line, the on-load tap-changer direct-current transmission harmonic current is judged whether to be over-current or not by detecting the real-time current change rate and comparing the real-time current change rate with the current change rate monitoring value, arc reignition of the on-load tap-changer is avoided, and the on-load tap-changer is protected and the on-off capability is ensured. And the on-load tap-changer on-off contact damage degree can be used for adaptively adjusting the monitoring value of the current change rate of the on-load tap-changer on-off contact, controlling the on-load tap-changer on-off capability is finer, fully controlling the use state of the on-load tap-changer is facilitated, the service life of the on-load tap-changer is prolonged, and accidents are reduced.
Example 2
Referring to fig. 6, another embodiment of the present invention is different from the first embodiment in that an experimental verification of a method for determining a harmonic current flowing through a dc power transmission is provided, and in order to verify and explain the technical effects adopted in the method, the embodiment adopts a conventional technical scheme to perform a comparison test with the method of the present invention, and the experimental results are compared by means of scientific demonstration to verify the true effects of the method.
According to the structural characteristics of the converter transformer vacuum on-load tap-changer, the switching process mainly comprises key parameters of transfer switching-on and switching-off current, power frequency recovery voltage, transfer switching-on current, switching-on voltage and the like, and the vacuum arc-extinguishing chamber is connected on-load tapThe most severe condition of the current required to be closed and opened in the switching process of the switch is superposition of rated load current and loop current in the switching process, and the most severe condition of the closing voltage required to be born by the vacuum arc extinguishing chamber in the switching process of the on-load tap changer and the recovery voltage after being opened is stage voltage and transition resistance R Transition Superposition of the voltages across. The test parameters for determining the harmonic current break test are as follows:
1) Test voltage: stage voltage + excess resistance x load current = 7560V;
2) Test current: 0.5× (load current+stage voltage/transition resistance) =1718a;
3) Number of tests: 3000 times;
4) Harmonic component selection: superposing 5 times and 7 times of harmonic currents, wherein the amplitude range of the 5 times and 7 times of harmonic currents is adjustable between 0 and 230A;
5) Test current di/dt value at turn-off time: 3A/us.
The used connection transformer is a single-phase double winding, the connection group is YN0YN0, the neutral point at the network side is directly grounded, the rated capacity is 375MVA, the rated voltage is (525/. Cndot.3+/-4 x 1.25%)/(375/. Cndot.3) kV, the rated current is IN=375×1000/525/. Cndot.3=1237A, therefore, the total harmonic content of the endurable load current is not more than 1237×5% =62A, the stage voltage of the on-load tap-changer is 3789V, and the rated current is 1302A.
The on-load tap-changer is divided into an A group and a B group, wherein the transition resistance value of the on-load tap-changer of the A group is n=0.55, and the transition resistance value of the on-load tap-changer of the B group is n=0.22.
According to the above analysis, the main contact arc current I, the recovery voltage Ur, the current rate of change di/dt are respectively as follows:
I＝1302A
Ur＝RI＝1.6×1302＝2083.2V
di/dt＝√2×1302×ω＝0.578A/us
in the on-off capacity test of the on-load tap-changer type test: the arc current, recovery voltage and di/dt values are respectively:
group A: i=4800 u=5136 di/dt=2.133
Group B: i=1840 u=7120di/dt= 0.817
As can be seen from the experimental results, the di/dt of group B is smaller than that of group A, i.e. the transition resistance R is reduced Transition The du/dt can be reduced and the recovery speed of the recovery voltage can be reduced to prevent the tap changer from being re-broken.
The load current of the converter transformer contains harmonic waves with complex components, so that the switching capacity of the on-load tap changer of the converter transformer must consider the influence of the harmonic waves. For an on-load tap changer of a converter transformer, besides the amplitude of the recovery voltage, the waveform of the recovery voltage has a decisive influence on the on-off capability. The recovery voltage rise across the main on-off contact of a resistive on-load tap changer operating in either the flag cycle or the multiple resistance cycle method is equal to the voltage drop across the transition resistor by the current, and therefore the recovery voltage is also a non-sinusoidal waveform. During the switching process of the on-load tap-changer, di/dt at the zero crossing of the load current is many times greater than that of the sinusoidal current, and the change speed of the recovery voltage is also many times greater.
The on-load tap-changer is to extinguish the arc at the first zero crossing point of the load current, and normally, the on-load tap-changer is designed by only considering the recovery voltage between the breaks when the sinusoidal fundamental wave current is considered, so as to ensure the reliable extinction of the fundamental wave current.
The actual operation condition of the converter transformer is non-ideal, and mainly comprises the following situations:
(1) The dc output current has ripple.
(2) The net side ac voltage has harmonic components, and the net side ac fundamental voltage is asymmetric due to the negative sequence voltage.
(3) The short-circuit impedance of each converter transformer is different.
(4) The trigger pulses of the y-joint converter valve and the d-joint converter valve are not equidistant and have different control angles.
Therefore, the network side current of the converter transformer has not only characteristic harmonic waves but also non-characteristic harmonic waves, and the on-load tap-changer of the converter transformer is required to have strong capacity of cutting off harmonic current.
It should be noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solution of the present invention may be modified or substituted without departing from the spirit and scope of the technical solution of the present invention, which is intended to be covered in the scope of the claims of the present invention.
Claims (5)
1. The method for judging the current flowing through the direct-current transmission harmonic current is characterized by comprising the following steps of:
establishing a harmonic break-off experimental model;
calculating the current change rate of the on-load tap-changer based on experimental data of the harmonic switching-off experimental model, and generating a first database and a second database based on the current change rate and the value of the control angle alpha;
calculating the current value of each control angle within the range of 0-90 DEG of the control angle alpha, and calculating the current change rate di/dt of the control angle alpha;
the calculation result of di/dt is in one-to-one correspondence with the control angle alpha, so that a first database is obtained;
under the dual resistance mode, calculating the current value of each control angle within the range of 0-90 degrees of the control angle alpha;
and calculate the current change rate di/dt respectively γ ；
Will di/dt γ The calculation results of the control angles are in one-to-one correspondence with the control angles alpha, so that a second database is obtained;
di/dt in a second database γ Subtracting di/dt from di/dt in the second database to obtain di/dt Δ ；
Will di/dt Δ The value is used as a current change rate margin value, and the current change rate margin value is correspondingly classified according to the size and the damage degree;
calculating a current change rate margin value;
judging whether the DC transmission harmonic current of the on-load tap-changer is over-current or not;
grading the damage degree of the on-off contact of the on-load tap-changer on the change current;
detecting and calculating the current change rate of the on-load tap-changer in real time during operation, and calculating the maximum current change rate under the maximum recovery voltage;
subtracting the current change rate margin value from the maximum current change rate to obtain a current change rate monitoring value;
comparing the value of the real-time current change rate with the monitored value of the current change rate;
and if the value of the real-time current change rate is larger than or equal to the monitoring value of the current change rate, triggering overcurrent early warning.
2. The direct-current transmission harmonic current passing current judgment method according to claim 1, wherein: the computational expression of di/dt of the on-load tap-changer is as follows:
I d ＝I
Ur＝RI
wherein di/dt represents the current change rate, I d The main contact arc current is represented by Ur, the recovery voltage is represented by R, the on-load tap-changer resistance is represented by I, the rated current of the on-load tap-changer is represented by I, and the control angle is represented by α.
3. The direct-current transmission harmonic current passing current judgment method according to claim 2, wherein: the recovery voltage rise on the main on-off contact of the on-load tap-changer is equal to the voltage drop generated at two ends of the transition resistor by current, when the recovery voltage is higher than the recovery rate of an insulating medium, the arc can reburns, the maximum recovery voltage is calculated, and the calculation expression is as follows:
maximum recovery voltage=r Transition ×(di/dt)；
Wherein R is Transition And the resistance value of the transition resistor is represented, and when the recovery voltage on the main on-off contact of the on-load tap-changer is lower than the maximum recovery voltage, the arc cannot be reburned.
4. The direct-current transmission harmonic current passing current judgment method according to claim 1, wherein:
a protection resistor is connected in series at one side of the on-load tap-changer, and the resistance value of the protection resistor is changed to simulate the resistance values of the on-load tap-changer in different states, and the maximum recovery voltage is calculated in a double-resistance mode at the moment, wherein the calculation expression is as follows;
Ur γ ＝IR protection of
Maximum recovery voltage Dual resistor ＝(di/dt γ +I(R-R Protection of ))/R Transition
Wherein di/dt γ Indicating the current change rate in the dual resistance mode, ur γ The recovery voltage in the double-resistance mode is represented by R, the on-load tap-changer resistor is represented by I, the rated current of the on-load tap-changer is represented by I, the control angle is represented by alpha, and the maximum recovery voltage is represented by alpha Dual resistor Represents the maximum recovery voltage in the double-resistance mode, R Protection of The resistance value of the protection resistor is represented, the resistance value of the protection resistor corresponds to the on-off capability adjustment of the main on-off contact of the on-load tap changer, and as the on-off capability of the main on-off contact of the on-load tap changer is reduced along with the reduction of arc discharge, the visual representation is that the on-off capability is more sensitive to di/dt, the resistance of the on-load tap changer is gradually reduced, the resistance values of the historical on-load tap changers with different degrees of arc discharge are measured, and the resistance value R of the historical on-load tap changer is obtained History of The relational expression is as follows:
R-R history of ＝R Protection of 。
5. The direct-current transmission harmonic current passing current judgment method according to claim 1, wherein:
the damage degree of the main on-off contact of the historical on-load tap-changer is graded, wherein the grading comprises slight damage, moderate damage, severe damage and severe damage.
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| CN112327151A (en) * | 2020-10-26 | 2021-02-05 | 南方电网科学研究院有限责任公司 | Method and device for monitoring switching state of on-load tap-changer and terminal equipment |
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