A method for evaluating the withstand voltage sag of AC contactors

Abstract

The invention relates to the technical field of power quality, in particular to an evaluation method for withstand voltage sag of an AC contactor, comprising the following steps: S1 characterizing a voltage sag waveform monitored in a power grid by using a time domain superposition form of a step signal sequence; S2 constructing a micro-variable equivalent model of the AC contactor according to the relationship between energy increments of various parts inside the AC contactor; S3 determining a working state of the AC contactor by comparing a spring compression amount of the AC contactor determined by the micro-variable equivalent model with a threshold value of the AC contactor; the invention can reflect the influence of an irregular sag waveform of an actual power grid on the working state of the AC contactor, and avoids the problem of excessive evaluation of irregular sags by traditional evaluation methods.

Description

Technical Field

[0001] The invention relates to the technical field of power quality, and in particular to a method for evaluating voltage sag withstand of an AC contactor. Background Art

[0002] As one of the typical power quality sensitive devices, AC contactor is easily affected by voltage sag and automatically trips, causing unplanned shutdown of industrial production and huge losses. Voltage sag includes rectangular wave sag and irregular sag. Researching an AC contactor tolerance performance evaluation method that can be applied to irregular sags can evaluate the degree to which various types of AC contactors within the monitoring range are affected by sags. This is an important prerequisite for accurately evaluating the economic losses caused by sags to users and provides an important basis for resolving economic compensation disputes between power suppliers and users.

[0003] The traditional method for evaluating the tolerance of AC contactors to irregular sags maps the points corresponding to the maximum sag amplitude and the longest duration onto the tolerance curve. The accuracy of the evaluation depends on the similarity between the actual sag waveform and the rectangular wave. When evaluating the tolerance performance of AC contactors to irregular sags, applying the traditional evaluation method will cause over-evaluation, thereby overestimating user losses. Summary of the invention

[0004] In view of the above-mentioned defects in the prior art, a method for evaluating the tolerance of irregular voltage sag of an AC contactor based on a micro-variable equivalent model is provided to solve the problem of over-evaluation of irregular voltage sag by traditional evaluation methods.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A method for evaluating the withstand voltage sag of an AC contactor comprises the following steps:

[0007] S1 characterizes the voltage sag waveform monitored in the power grid by using the time domain superposition form of a step signal sequence;

[0008] S2 constructs a micro-variable equivalent model of the AC contactor based on the relationship between the energy increments of various parts inside the AC contactor;

[0009] S3 determines the working state of the AC contactor by comparing the AC contactor spring compression determined by the micro-variable equivalent model with the AC contactor threshold.

[0010] The further improvement of the technical solution of the present invention is that: the characterization of the voltage sag in S1 is specifically: setting a small sampling interval ΔT, setting the kth sampling interval to t k ~t k+1 The voltage of the kth interval is the sampling time t k The step voltage u k, and remains unchanged in a small sampling interval, n is the maximum sampling number of the sag, and the voltage sag waveform can be regarded as a step signal sequence (u1(t), u2(t), ..., u n (t)) is a superposition form of a small change in the time domain. The voltage sag waveform is sampled, and the voltage step signal sequence expression obtained is:

[0011]

[0012] A further improvement of the technical solution of the present invention is that: S2 includes the following steps: S21, calculating the coil current i of the kth sampling interval k With inductance L k When k=1, the sag starts at time t1. Since the current cannot change suddenly, the current value at this time can be solved based on the voltage and impedance before the sag. The expression is:

[0013]

[0014] Among them, U m is the voltage amplitude before the sag; α is the starting angle of the voltage sag; R is the equivalent resistance of the coil circuit, L0 is the inductance of the main coil before the temporary drop, and the expression is:

[0015]

[0016] Where N is the number of turns of the main coil; μ0 is the magnetic permeability of vacuum; μ r is the relative magnetic permeability of the core; S is the cross-sectional area of ​​the main coil; l e is the magnetic circuit length. At the beginning of the kth micro-sampling interval, the voltage changes stepwise from U k-1 Mutation to U k , the inductance current of the main coil changes from the initial value i k(-) According to the exponential law, the current value after the sampling interval ΔT becomes i k(+) , the expression is:

[0017]

[0018] Among them, i k(-) 、i k(+) Respectively represent the current value of the coil at the start and end of the kth sampling interval, and i k(-) =i k-1(+) , take i k(+) As the kth slightly changing time domain current i k τ=R / L k is the time constant of the kth sampling interval; For U k The steady-state value of the coil current under the action of Lk is the main coil inductance of the kth sampling interval, and the expression is:

[0019]

[0020] S22, calculate the energy increments in each micro-variation time domain of the AC contactor. In the kth micro-variation time domain, the energy increment ΔW input by the power supply to the coil elec,k , the energy increment ΔW consumed by the coil resistance r,k And the magnetic field energy increment ΔW stored in the coil f,k The expressions are:

[0021]

[0022] S23. The k-th slightly changed time domain energy increment relationship of the AC contactor satisfies the law of conservation of energy, and the expression is:

[0023] ΔW mech,k =-(ΔW elec,k +ΔW r,k +ΔW f,k )

[0024] Where, ΔW mech,k It represents the increment of the total mechanical energy of the contactor in the kth micro-variation time domain.

[0025] S24, the increment ΔW of the total mechanical energy of the contactor in the kth micro-variation time domain mech,k for

[0026] ΔW mech,k =ΔW F,k +ΔW v,k

[0027] Where, ΔW F,k represents the increment of the elastic potential energy of the spring in the kth micro-variation time domain, b is the equivalent elastic coefficient of the spring, x k(-) 、x k(+) are the spring compression at the start and end of the kth sampling interval, respectively, and x k(-) =x k-1(+) , ΔW v,k represents the kinetic energy increment of the moving iron core and its linkage parts in the kth micro-variation time domain, m is the mass of the moving iron core and its linkage parts, v is k(-) 、v k(+) are the speeds of the moving iron core and its linkage parts at the start and end of the kth sampling interval, respectively, and v k(-) =v k-1(+) , in the kth sampling interval ΔT, the average speed of the moving iron core and its linkage parts is:

[0028]

[0029] Ignoring the speed change of the moving iron core and its linkage parts in the micro-time domain, let but

[0030] S25, substituting each energy increment expression into the energy increment relationship expression to obtain the slight change equivalent model, the expression is:

[0031]

[0032] In the expression of the slightly changed equivalent model, the meaning of each letter is indicated in the above steps;

[0033] The further improvement of the technical solution of the present invention is that: in S3, the determination of the working state of the AC contactor is specifically: calculating the compression amount x of the AC contactor spring k and the spring compression threshold x of the alternating current contactor min Compare. a)x k ≤x min , indicating that the AC contactor trips in the kth micro-variation time domain; b) x k >x min , indicating that the AC contactor maintains the energized state, k=k+1, the next sampling interval parameter is updated, and step S2 is repeated until the temporary drop ends.

[0034] Compared with the prior art, the present invention provides an AC contactor withstand voltage sag evaluation method with the following beneficial effects:

[0035] 1. The present invention proposes a method for evaluating the voltage sag tolerance of an AC contactor. According to the action mechanism of the AC contactor, an equivalent model of the AC contactor in a slightly variable time domain is constructed based on the energy relationship. The irregular voltage sag is characterized by the time domain superposition form of a step signal sequence. The superposition effect of each step response in the slightly variable time domain is solved in an iterative manner to evaluate the tolerance of the AC contactor under irregular voltage sag. The evaluation method provided by the present application effectively solves the problems of solving the main coil current, calculating the spring compression amount, and evaluating the tolerance capacity when the AC contactor is subjected to irregular voltage sag, and effectively avoids the excessive evaluation of irregular voltage sag by traditional tolerance performance evaluation methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0037] Figure 1 It is a schematic diagram of the structure of an AC contactor provided by an embodiment of the present invention;

[0038] Figure 2 is a schematic diagram of voltage waveform characterization provided by an embodiment of the present invention;

[0039] Figure 3 is an evaluation flow chart provided by an embodiment of the present invention;

[0040] Figure 4 is a schematic diagram of a simulated voltage sag waveform provided by an embodiment of the present invention;

[0041] Figure 5 It is a schematic diagram of the change of spring compression amount provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with embodiments:

[0043] like Figure 1 As shown in the figure, the structure of the AC contactor includes: a static iron core, a moving iron core and its linkage part and linkage part, a coil, a short-circuit ring, a spring, a moving contact and a static contact; after the power supply voltage is applied to both ends of the main coil, the alternating current generates an alternating magnetic flux, which causes the static iron core to generate an electromagnetic attraction, which attracts the moving iron core and drives the spring to compress, so that the moving and static contacts are attracted. When the voltage amplitude of the main coil remains unchanged, the AC contactor is in a fully attracted state, and the electromagnetic force keeps the spring compression at the maximum value x min When the coil is powered off, the electromagnetic attraction disappears, and under the action of the spring force, the moving and static contacts separate, the AC contactor jumps out of the attracted state, and the spring compression drops to 0.

[0044] S1, using the superposition form of step signals to characterize the voltage sag waveform;

[0045] S2, constructing the AC contactor micro-change equivalent model based on the relationship between energy increments;

[0046] S3, judging the working state of the AC contactor by comparing the AC contactor spring compression determined by the micro-variable equivalent model with the AC contactor threshold.

[0047] For S1, first set a sufficiently small sampling interval ΔT, and let the kth sampling interval be t k ~t k+1 The voltage of the kth interval is the sampling time t k The step voltage u k , and remains unchanged in a small sampling interval, n is the maximum sampling number of the voltage sag. The voltage sag waveform can be regarded as a step signal sequence (u1(t), u2(t), ..., u n(t)) in the superposition form of a small change in the time domain, such as Figure 2 As shown in the figure, the irregular voltage sag waveform can be regarded as an amplitude of U in the kth micro-change time domain. k , a rectangular wave with a duration of ΔT. The voltage sag waveform is sampled, and the voltage step signal sequence expression obtained is:

[0048]

[0049] For S2, the AC contactor main coil current at each sampling moment is a necessary parameter for calculating the incremental energy of each part. When a voltage drop occurs, the AC contactor main coil can be represented by a resistance-inductance loop with a resistor and an inductor in series. k(-) 、i k(+) Respectively represent the current value of the coil at the start and end of the kth sampling interval, and i k(-) =i k-1(+) , take i k(+) As the kth slightly changing time domain current i k Since the inductor current cannot change suddenly, the current value at the start of the sag t1 can be solved based on the voltage and impedance before the sag. This current is also the starting current of the first sampling interval, as shown in formula (2):

[0050]

[0051] Where: U m is the voltage amplitude before the sag; α is the starting angle of the voltage sag; R is the equivalent resistance of the coil loop, and L0 is the inductance of the main coil before the temporary drop, which can be expressed as:

[0052]

[0053] Where: N is the number of turns of the main coil; μ0 is the magnetic permeability of vacuum; μ r is the relative magnetic permeability of the core; S is the cross-sectional area of ​​the main coil; l e is the magnetic path length.

[0054] During the sag, the small changes in the sag amplitude in each micro-variable unit are ignored, and the entire voltage sag is represented by a number of step signals in the micro-variable time domain. The main coil current in each sampling interval can be regarded as a step response under the excitation of each voltage step signal. At the beginning of the kth micro-sampling interval, the voltage undergoes a step change, which is represented by U k-1 Mutation to U k , the inductance current of the main coil changes from the initial value i k(-) According to the exponential law, the current value after the sampling interval ΔT is:

[0055]

[0056] Where: τ = R / L k is the time constant of the kth sampling interval; For U k The steady-state value of the coil current under the action of is expressed as:

[0057]

[0058] L k is the main coil inductance of the kth sampling interval, which is related to the sampling voltage and current. The relationship is:

[0059]

[0060] By combining equations (4) and (6), the coil loop current and inductance in each sampling interval can be calculated.

[0061] After the irregular voltage sag waveform is characterized by the time domain superposition form of the step signal sequence and the coil current and inductance value of each sampling interval are calculated, the energy increment ΔW input by the power supply to the coil in the kth micro-change time domain is elec,k , the energy increment ΔW consumed by the coil resistance r,k And the magnetic field energy increment ΔW stored in the coil f,k The expressions are:

[0062]

[0063] For AC contactors, when voltage drops occur, the change in external characteristics is a direct reflection of the internal energy conversion. Considering the AC contactor with power supply as a whole, the sum of the energy changes of each part of the system must satisfy the law of conservation of energy, that is, the total energy of the system remains unchanged, and only the energy form is converted within the system. In the kth micro-change time domain, the energy increment ΔW input by the power supply to the coil is elec,k , the energy increment ΔW consumed by the coil resistance r,k , the magnetic field energy increment ΔW stored in the coil f,k And the increase of the total mechanical energy of the contactor ΔW mech,k The relationship between is shown in formula (8):

[0064] ΔW elec,k +ΔW r,k +ΔW f,k +ΔW mech,k =0 (8)

[0065] Total mechanical energy increment of AC contactor ΔW mech,k Contains the elastic potential energy increment ΔW caused by the change in spring compression F,k The kinetic energy increment ΔW generated by the movement of the moving iron core and its linkage parts v,k, as shown in formula (9):

[0066] ΔW mech,k =ΔW F,k +ΔW v,k (9)

[0067] In the kth sampling interval ΔT, at u k Under the action of the step signal, the main coil current and magnetic flux change, and the spring compression amount increases from x at the starting time to k(-) becomes x at the end time k(+) , the elastic potential energy of the spring produces an increment ΔW F,k , which can be expressed as:

[0068]

[0069] Where: b is the equivalent elastic coefficient of the spring; x k(-) =x k-1(+) .

[0070] Under the action of the changing electromagnetic force and spring force, the AC contactor spring has different degrees of compression changes in each sampling interval. The different degrees of compression change will cause the moving speed of the moving iron core and its linkage parts to be different, so the kinetic energy increment of the moving iron core and its linkage parts at different times will also be different. In the kth sampling interval, the moving iron core and its linkage parts have a speed v at the starting time. k(-) After ΔT, it becomes v k(+) , the kinetic energy increment is:

[0071]

[0072] Where: m is the mass of the moving iron core and its linkage parts; v k(-) =v k-1(+) .

[0073] The speed of the moving iron core and its linkage part satisfies the differential relationship with the spring compression. In the kth sampling interval ΔT, the average speed of the moving iron core and its linkage part is:

[0074]

[0075] Ignoring the speed change of the moving iron core and its linkage parts in the micro-time domain, let but

[0076] Substituting the elastic potential energy increment model of formula (10) and the core kinetic energy increment model of formula (11) into formula (9) can obtain the total mechanical energy increment model. Substituting the total mechanical energy increment model and the micro-change model of formula (7) into formula (8) can establish the AC contactor micro-change equivalent model, and the expression is:

[0077]

[0078] According to equations (12) and (13), the spring compression amount x of the AC contactor in the kth micro-variation time domain can be obtained: k(+) .

[0079] For S3, the spring compression amount x of the AC contactor in the kth micro-variation time domain is obtained k Then, according to the AC contactor spring compression threshold x min , the specific judgment logic is: a) x k ≤x min , indicating that the AC contactor trips in the kth micro-change time domain; b) x k >x min , indicating that the AC contactor maintains the energized state, k=k+1, the next sampling interval parameter is updated, and step S2 is repeated until the temporary drop ends.

[0080] The difference from the existing AC contactor withstand voltage sag performance evaluation method is:

[0081] Existing research has not analyzed the impact of voltage sag from the perspective of the AC contactor mechanism. It is necessary to first obtain the voltage tolerance curve of this model of AC contactor through experimental means, and then use the maximum amplitude and longest duration points of the voltage sag waveform as a benchmark to map it to the voltage tolerance curve, and directly amplify the irregular voltage sag waveform into a rectangular sag waveform with the maximum sag amplitude and the longest duration, which over-evaluates the impact of irregular sags on AC contactors. Therefore, it is only suitable for evaluating rectangular sags but not irregular sags.

[0082] The present invention proposes a method for evaluating the withstand voltage sag of an AC contactor. A micro-variation equivalent model of the AC contactor is established based on the action mechanism of the AC contactor. The spring compression of the AC contactor under the action of the voltage sag can be directly obtained without conducting experiments. The calculation result is accurate, and the influence of irregular voltage sag on the AC contactor can be accurately evaluated.

[0083] In summary, the evaluation method proposed in the present invention has a wider application range and is applicable to voltage sags of arbitrary waveforms. For irregular voltage sag waveforms, the working state of the AC contactor can be judged more accurately. In addition, the method proposed in the present invention can also calculate the disconnection time of the AC contactor.

[0084] like Figure 4As shown in the figure, it is a schematic diagram of the simulated sag waveform applied to the evaluation of the irregular sag tolerance of the AC contactor according to the present invention. The voltage sag amplitudes marked in the legend are the minimum values ​​of the voltage root mean square during the sag period. The selected AC contactor model is Chint NC3-45, and the parameters are as follows: rated voltage 220V, rated frequency 50Hz, coil 3300 turns, moving iron core and its linkage part mass 0.014kg, vacuum permeability μ0 is 4π×10 -7 H / m, relative magnetic permeability of the core μ r 8000H / m, magnetic circuit length l e The cross-sectional area of ​​the main coil is 0.082 m, and the cross-sectional area of ​​the main coil is 3.04×10 -4 m 2 , the spring elastic coefficient is 2884N / m and the maximum compression length is 9mm. Figure 4 The AC contactor spring compression corresponding to the temporary drop waveform is as follows: Figure 5 shown.

[0085] The embodiments described above are merely descriptions of preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the scope of protection determined by the claims of the device of the present invention.

Claims

1. A method for evaluating the withstand voltage sag of an AC contactor, characterized in that: The steps include: S1. The voltage sag waveform monitored in the power grid is characterized by a time domain superposition form of a step signal sequence; S2. According to the relationship between the energy increments of various parts inside the AC contactor, a micro-variable equivalent model of the AC contactor is constructed; S3, comparing the AC contactor spring compression determined by the micro-variable equivalent model with the AC contactor threshold to determine the working state of the AC contactor; In S1, a step signal is used to characterize the voltage sag waveform, including: setting the sampling interval to ΔT, the kth sampling interval to t k ~t k+1 The voltage of the kth interval is the sampling time t k The step voltage u k , and remains unchanged within a small sampling interval; the voltage sag waveform is characterized by the time domain superposition of a step signal sequence. The voltage sag waveform can be regarded as a series of step signals (u1(t), u2(t), ..., u n (t)) in the form of superposition on a slightly varying time domain; In S2, building a micro-variable equivalent model of the AC contactor includes the following steps: S21, calculate the AC contactor coil current i respectively k(+) With inductance L k ; S22, the energy increment ΔW input from the power supply to the coil in the micro-variable equivalent model elec,k , the energy increment consumed by the coil resistance ΔW r,k , the magnetic field energy increment ΔW stored in the coil f,k The expression is In the formula, R represents the equivalent resistance of the coil loop, i k(-) 、i k(+) Respectively represent the coil current at the start and end time of the kth micro-variation time domain; S23. The energy conservation equation satisfying the construction of the micro-variable equivalent model is: ΔW mech,k =-(ΔW elec,k +ΔW r,k +ΔW f,k ) In the formula, ΔW mech,k represents the increment of total mechanical energy in the kth micro-variation time domain; S24, the increment ΔW of the total mechanical energy of the contactor in the kth micro-variation time domain mech,k for ΔW mech,k =ΔW F,k +ΔW v,k In the formula, ΔW F,k It represents the increment of the elastic potential energy of the spring in the kth micro-variation time domain, ΔW v,k It represents the kinetic energy increment of the moving iron core and its linkage parts in the kth micro-variation time domain; S25, constructing the AC contactor micro-variable equivalent model; In S25, the expression of the AC contactor micro-transformer equivalent model is:

2. According to claim 1, a method for evaluating the withstand voltage sag of an AC contactor, characterized in that: In S21, the current i k(+) With inductance L k The expression is Where, τ = R / L k is the time constant of the kth sampling interval; for u k The steady-state value of the coil current under the action of 3. The method for evaluating the withstand voltage sag of an AC contactor according to claim 1, characterized in that: In S24, the elastic potential energy of the spring generates an increment ΔW F,k The kinetic energy increment ΔW of the moving iron core and linkage part in the micro-variation time domain v,k The expression is In the formula, b is the spring elastic coefficient, m is the mass of the moving iron core and its linkage part, and v is k(-) 、v k(+) They respectively represent the speed of the moving iron core and its linkage part at the starting and ending moments of the kth slight change time domain.

4. The method for evaluating the withstand voltage sag of an AC contactor according to claim 1, characterized in that: In S24, the spring compression amount and the speed of the moving iron core and its linkage part satisfy the following relationship: In the formula, is the average speed of the moving iron core and its linkage parts in the kth micro-variation time domain, ignoring the speed change in the micro-variation time domain, and let but x k(-) 、x k(+) They respectively represent the spring compression at the start and end times of the kth slight change time domain.

5. The method for evaluating the withstand voltage sag of an AC contactor according to claim 1, characterized in that: In S3, the determination of the working state of the AC contactor is as follows: Calculate the spring compression amount x of the AC contactor k and the spring compression threshold x of the alternating current contactor min contrast; a)x k ≤x min , indicating that the AC contactor trips in the kth micro-variation time domain; b) x k >x min , indicating that the AC contactor maintains the energized state, k=k+1, the next sampling interval parameter is updated, and step S2 is repeated until the temporary drop ends.

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