A fault direction discrimination method suitable for energy storage power station access to distribution network

Abstract

The present application relates to the protection technical field of energy storage power station access distribution network, especially to a kind of fault direction discrimination method suitable for energy storage power station access distribution network.The technical scheme includes the comparative analysis of the transient process mechanism of fault current of synchronous power supply and energy storage power station, the obvious difference of the transient component of fault current of both is obtained, namely the decay time constant of the decay DC component in the fault current of synchronous power supply is related to line parameter, the decay time constant of the decay DC component in the fault current of energy storage power station is related to its control parameter and filter parameter;The sampled fault current is processed twice;Set fluctuation range, and the change rate of the data after twice data processing is detected.The present application discriminates fault direction using the difference of transient characteristics of fault current of synchronous power supply and energy storage power station, is not influenced by steady-state control strategy of energy storage power station, is not influenced by charge-discharge state of energy storage power station, and has clear directionality.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of protection of energy storage power station accessing distribution network, and particularly relates to a fault direction discrimination method suitable for energy storage power station accessing distribution network. BACKGROUND

[0002] The proportion of distributed or centralized new energy accessing distribution network is increasing in China at present, however, the output of new energy has the problems of randomness and fluctuation, and the energy storage power station plays an important role in power fluctuation suppression, voltage support and new energy consumption enhancement, so the scenario of energy storage power station accessing distribution network will be more common. The access of new energy and energy storage makes the distribution network change from a simple radial to a complex network with double-end or multi-end power supply, and in this scenario, whether the directional element in the directional current protection in the distribution network can correctly act is crucial. Since the energy storage power station is an inverter type power supply, on the one hand, its control strategy is flexible and changeable, so that the fault current presents the characteristics of amplitude limitation and phase control; on the other hand, unlike wind power and photovoltaic power generation, the existence of the charging state of the energy storage power station makes the phase variation range of the fault current larger, and there is the characteristic of reverse power flow. The fault characteristics of the energy storage power station are obviously different from those of the traditional synchronous power supply, so the traditional directional elements such as the phasor power directional element, the sequence component power directional element and the sudden change directional element are difficult to be applicable in the scenario of energy storage power station accessing, and therefore it is necessary to study a new fault direction discrimination method suitable for the scenario of energy storage power station accessing distribution network.

[0003] Therefore, the present application provides a fault direction discrimination method suitable for energy storage power station accessing distribution network. SUMMARY

[0004] The purpose of the present application is to solve the problem that the traditional directional elements cannot correctly act, causing directional misjudgment and protection misoperation or refusal in the scenario of energy storage power station accessing distribution network, and a fault direction discrimination method suitable for energy storage power station accessing distribution network is provided.

[0005] The technical scheme of the present application: a fault direction discrimination method suitable for energy storage power station accessing distribution network, the transient process mechanism of the fault current of the synchronous power supply and the energy storage power station is compared and analyzed, and it is obtained that the transient components of the fault current of the two are obviously different, that is, the decay time constant of the decay DC component in the fault current of the synchronous power supply is related to the line parameter, and the decay time constant of the decay DC component in the fault current of the energy storage power station is related to the control parameter and the filter parameter thereof;

[0006] The sampled fault current undergoes two data processing steps: After the first data processing, if the fault current is supplied by a synchronous power source, it will only contain steady-state power frequency quantities. If the fault current is supplied by an energy storage station, it will still contain attenuated power frequency, harmonic, and steady-state power frequency quantities, with the proportion of attenuated harmonic quantities increasing. After the second data processing, if the fault current is supplied by a synchronous power source, it will become a linear current; if supplied by an energy storage station, it will be a fluctuating current.

[0007] Set a fluctuation range and perform a rate of change detection on the data after two data processing steps. That is, determine whether the rate of change of several consecutive data points is within the set range. If it is within the set range, it is determined that the fault current is provided by the synchronous power supply; otherwise, it is determined that the fault current is provided by the energy storage power station.

[0008] Optionally, the fault current of the first data processing is differentiated, multiplied by a fixed coefficient, and the difference is calculated. This coefficient is determined by the line parameters and is a fixed value.

[0009] The fault current obtained from the second data processing is differentiated again, divided by the power frequency angular velocity, and then summed with the squares of the result obtained from the first data processing.

[0010] Optionally, the fault current of the synchronous power supply can be expressed as:

[0011]

[0012] Among them, A g The steady-state power frequency component amplitude; ω1 is the initial phase of the steady-state power frequency component; C is the power frequency angular velocity; g To attenuate the amplitude of the DC component; τ g 1 / τ is the decay time constant. g =R / L, the magnitude of which is related to the line parameters and the internal resistance of the system; t is time; e is the natural base.

[0013] Optionally, to eliminate the attenuated DC component in the synchronous power supply fault current, the derivative of the synchronous power supply fault current is obtained as follows:

[0014] Optionally, by simultaneously solving the fault current equations before and after differentiation, the derivative of the fault current of the synchronous power supply is multiplied by the decay time constant τ. g And add it to the fault current of the synchronous power supply to get i g.r1 (t), represented as:

[0015]

[0016] Optionally, for i g.r1 (t) After the second data processing, i is obtainedg.r2 (t), is expressed as:

[0017]

[0018] wherein i' g.r1 (t) is i g.r1 (t) is the derivative of i

[0019] Optionally, the fault current provided by the energy storage power station is expressed as:

[0020]

[0021] wherein A is the amplitude of the steady-state power frequency component; is the initial phase of the steady-state power frequency component; ω1 is the angular velocity of the power frequency; Nω1 is the angular velocity of the decay multiple frequency component, B N is the amplitude of the decay multiple frequency component, is the initial phase of the decay multiple frequency component, τ N is the decay time constant of the decay multiple frequency component.

[0022] Optionally, the derivative of the fault current provided by the energy storage power station is obtained as:

[0023]

[0024] Optionally, the derivative value of the fault current provided by the energy storage power station is multiplied by the decay time constant τ g and added to the fault current provided by the energy storage power station to obtain i b.r1 (t), is expressed as:

[0025]

[0026] Optionally, the second data processing is performed on i b.r1 (t) to obtain i b.r2 (t), is expressed as:

[0027]

[0028] wherein i' b.r1 (t) is i b.r1 (t) is the derivative of i

[0029] Compared with the prior art, the present application has the following beneficial technical effects:

[0030] The application utilizes the difference between the transient characteristics of the fault current of the synchronous power supply and the energy storage power station, amplifies the difference of the detected fault current through certain data processing, and then quickly and accurately judges whether the fault current comes from the synchronous power supply side or the energy storage power station side, so as to realize the discrimination of the fault direction, which is not affected by the steady-state control strategy of the energy storage power station and the charging and discharging state of the energy storage power station, and has clear directionality. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 Schematic diagram for solving synchronous power supply fault current;

[0032] Figure 2 Schematic diagram for energy storage power station accessing distribution network;

[0033] Figure 3 Mechanism analysis diagram for transient process of fault current of energy storage power station;

[0034] Figure 4 Control block diagram of current loop closed loop system. DETAILED DESCRIPTION

[0035] The technical solutions of the application will be further described below in combination with the drawings and specific embodiments.

[0036] EMBODIMENT

[0037] The application proposes a fault direction discrimination method in the scenario of energy storage power station accessing distribution network. After a short-circuit fault occurs in the line, the synchronous power supply after the fault can be equivalent to a voltage source in series with internal impedance, and the time-domain expression of the fault current provided by the synchronous power supply can be obtained from the differential equation of the line from the internal potential segment of the power supply to the fault point.

[0038] When a short-circuit fault occurs in the line, take phase a as an example, as shown in the figure. Figure 1 If the internal impedance of the power supply is ignored, the voltage equation from the internal potential of phase a to the fault point is as follows formula (1). Wherein u a is the internal potential of system phase a; U m is the amplitude of the internal potential of phase a; U fm is the amplitude of the fault point voltage of phase a; α is the phase of the fault point voltage of phase a; i a is the current of phase a; U m is the amplitude of the voltage drop from the internal potential of phase a to the fault point; β is the phase of the voltage drop from the internal potential of phase a to the fault point; R is the equivalent resistance of the line from the power supply of phase a to the fault point; L is the equivalent inductance of the line from the power supply of phase a to the fault point; ω is the angular velocity of power frequency; d represents differentiation; t is time.

[0039]

[0040] The particular solution i pa of the first-order constant coefficient linear non-homogeneous differential equation is:

[0041]

[0042] In formula (2), is

[0043] General solution of differential equation i αa where C is a constant, e is the natural base number:

[0044]

[0045] According to the fact that the inductance current cannot be suddenly changed before and after the fault, the following can be obtained:

[0046]

[0047] where I m0 is the amplitude of the a-phase current before the fault; t f is the time of the fault, is the phase angle of the a-phase current lagging behind the a-phase potential before the fault. Then the time-domain expression of the a-phase short-circuit current after the fault is:

[0048]

[0049] In formula (5), i pa is the particular solution; and i αa is the general solution. As can be seen from formula (5), the short-circuit current provided by the synchronous power source contains a steady-state power frequency component and a decaying DC component, and the initial value of the decaying DC component is determined by the time of the fault and the voltage drop at the fault point, and can be zero. Therefore, the time-domain expression of the short-circuit current provided by the synchronous power source in the three-phase stationary coordinate system is as follows:

[0050]

[0051] In the formula, A g is the amplitude of the steady-state power frequency component; is the initial phase of the steady-state power frequency component; ω1 is the power frequency angular velocity; C g is the amplitude of the decaying DC component; τ g is the decay time constant, and 1 / τ g =R / L, which is related to the line parameters and the system resistance. Since the differential equation written when calculating the time-domain expression of the fault current is a first-order constant coefficient linear non-homogeneous differential equation, the solution of the equation has relatively simple components, only contains a power frequency steady-state component and a decaying DC component, and the amplitude of the decaying DC component is affected by the time of the short circuit, and can be zero.

[0052] Please refer to Figures 2-4The energy storage power station belongs to an inverter type power supply, and its fault current is affected by the dynamic response of each link of the control system, including the dynamic response of the phase-locked loop, the dynamic response of the positive and negative sequence decomposition link, and the PI control response. That is, the positive and negative sequence quantities of the grid voltage and current introduced into the current loop control equation during the dynamic response period are not the actual values, and there is a certain deviation. This deviation introduced into the current loop control system on the one hand makes the dq axis quantities in the current loop no longer decoupled, and the dq axis voltage and current exist cross-coupling effects; on the other hand, due to the existence of the positive and negative sequence decomposition link transfer function, the order of the closed-loop system is increased, and the transient response is more difficult and complex to solve. Ultimately, it is reflected in the extension of the transient process of the fault current and the obvious nonlinear characteristics. Taking the solution of the d-axis current response as an example, the current loop closed-loop system is Figure 3 According to the transfer function of each link, the closed-loop transfer function of the current loop can be obtained, and then the fault current response is obtained. However, due to the existence of multiple link transfer functions, the current loop closed-loop system is a high-order coupled system, and the fault current time domain expression obtained after Laplace inverse transformation is complex, containing steady-state power frequency components, decaying power frequency components, and decaying frequency doubling components. The time domain expression of the a-phase short-circuit current provided by it is:

[0053]

[0054] In the formula, A is the amplitude of the steady-state power frequency component; is the initial phase of the steady-state power frequency component;

[0055] Based on the differences in the transient characteristics of the fault current of the synchronous power supply and the energy storage power station, the detected fault current is processed and amplified to enlarge the differences, and then it is judged whether the fault current comes from the synchronous power supply side or the energy storage power station side, so as to realize the discrimination of the fault direction.

[0056] Specifically, the fault currents provided by the synchronous power supply and the energy storage power station are rewritten as the following formula respectively:

[0057]

[0058] In order to eliminate the decaying DC component in the fault current of the synchronous power supply, the derivatives of the fault current provided by the synchronous power supply in formula (8) and the fault current provided by the energy storage power station in formula (9) are obtained respectively.

[0059]

[0060] As can be seen from formulas (10) and (11), after derivation, the amplitude of the decaying frequency doubling component is increased to Nω1 times of the original, the amplitude of the steady-state power frequency component is only increased to ω1 times, and the amplitude of the DC decaying component will be increased to 1 / τ g times.

[0061] Further, the fault current before and after the simultaneous derivation is multiplied by the decay time constant τ of formula (10) g And formula (8) is added to obtain formula (12), denoted as i g.r1 (t); and formula (11) is processed in the same way to obtain formula (13), denoted as i b.r1 (t).

[0062]

[0063] Theoretically, i g.r1 (t) does not contain the decay DC component, only contains the steady-state power frequency component; and the proportion of the decay DC component and the decay frequency component in i b.r1 (t) is increased, and the harmonic characteristics are more obvious. After the first data processing, the difference between the fault current provided by the system and the fault current provided by the inverter power supply is significantly amplified.

[0064] In order to fully utilize the characteristics of i g.r1 (t) is a power frequency steady-state component, improve the fault direction discrimination speed and reliability, the following needs to be processed twice the results obtained by the first data processing. At this time, the fault current provided by the synchronous power supply has been changed into a power frequency sine signal after the first data processing, if it is further derived and the trigonometric function properties are used, it can be further constructed into a direct current, and the second derivation will further amplify the amplitude of the high frequency harmonic component in the fault current of the energy storage power station.

[0065] Using formula (14), (15) to further process i g.r1 (t) to obtain i g.r2 (t), and processing i b.r1 (t) in the same way to obtain i b.r2 (t), wherein i' g.r1 (t), i' b.r1 (t) are the derivatives of i g.r1 (t), i b.r1 (t).

[0066]

[0067] After the second data processing, the fault current provided by the synchronous power supply is constructed into a direct current; the fault current provided by the inverter power supply is a fluctuating quantity with more complex harmonic content and larger proportion.

[0068] In practical application, the sampled current is processed as described above, the result obtained by processing is subjected to rate of change determination, if the value of the next sampling point is outside 0.5-1.5 times of the value of the previous sampling point, the logic output is 0, otherwise the logic output is 1. The number of 1 in 20 logic output values after determining the fault is judged, if the number of 1 is greater than or equal to 15, it is determined that the fault current is provided by the synchronous power supply, otherwise it is determined that the fault current is provided by the energy storage power station, and then the direction of the fault occurrence is determined.

[0069] Since the derivation operation is performed in the data processing process, too high sampling rate will make the noise signal obvious; too low sampling rate will make the effective information in the fault current lost, through test, better effect can be obtained when the sampling rate is 4 kHz.

[0070] The above specific embodiments are only several optional embodiments of the present application, based on the technical scheme of the present application and the related inspiration of the above embodiments, the person skilled in the art can make various alternative improvements and combinations on the above specific embodiments.

Claims

1. A fault direction determination method applicable to energy storage power stations connected to distribution networks, characterized in that, include: A comparative analysis of the fault current transient process mechanism of synchronous power supply and energy storage power station reveals that there are significant differences in the fault current transient components of the two. Specifically, the decay time constant of the decaying DC component in the fault current of synchronous power supply is related to the line parameters, while the decay time constant of the decaying DC component in the fault current of energy storage power station is related to its control parameters and filter parameters. The sampled fault current undergoes two data processing steps: After the first data processing, if the fault current is supplied by a synchronous power source, it will only contain the power frequency steady-state quantity; if the fault current is supplied by an energy storage power station, it will still contain attenuated power frequency, harmonic frequency, and power frequency steady-state quantities, with the proportion of attenuated harmonic frequency quantities increasing; after the second data processing, if the fault current is supplied by a synchronous power source, it will become a DC current. If supplied by an energy storage power station, then it is a fluctuating quantity; Set a fluctuation range and perform a rate of change detection on the data after two data processing steps. That is, determine whether the rate of change of several consecutive data points is within the set range. If it is within the set range, it is determined that the fault current is provided by the synchronous power supply; otherwise, it is determined that the fault current is provided by the energy storage power station. The fault current of the first data processing is differentiated, multiplied by a fixed coefficient, and the difference is calculated. This coefficient is determined by the line parameters and is a fixed value. The fault current obtained from the second data processing is differentiated again, divided by the power frequency angular velocity, and then summed with the squares of the result obtained from the first data processing.

2. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 1, characterized in that, The fault current of the synchronous power supply is expressed as: ； in, The steady-state power frequency component amplitude; This represents the initial phase of the steady-state power frequency component. It is the angular velocity at power frequency; To attenuate the amplitude of the DC component; The decay time constant, Its size is related to the line parameters and the internal resistance of the system. For time; It is the natural base.

3. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 2, characterized in that, To eliminate the attenuated DC component in the fault current of the synchronous power supply, the derivative of the fault current of the synchronous power supply is obtained as follows: 。 4. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 3, characterized in that, Solve the equations for the fault current before and after differentiation, then multiply the derivative of the fault current of the synchronous power supply by the decay time constant. And add it to the fault current of the synchronous power supply to get , is represented as: 。 5. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 4, characterized in that, right After the second data processing, we obtain , is represented as: ； In the formula, for The derivative of .

6. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 1, characterized in that, The fault current provided by the energy storage power station is expressed as: ； in, The steady-state power frequency component amplitude; This represents the initial phase of the steady-state power frequency component. To attenuate the angular velocity of the harmonic component, To attenuate the amplitude of the harmonic component, To attenuate the initial phase of the harmonic component, This is the attenuation time constant for the attenuation harmonic component.

7. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 6, characterized in that, The derivative of the fault current provided by the energy storage power station is: 。 8. The fault direction determination method applicable to energy storage power stations connected to the distribution network according to claim 7, characterized in that, Multiply the derivative of the fault current provided by the energy storage power station by the decay time constant. And add it to the fault current provided by the energy storage power station to obtain , is represented as: 。 9. A fault direction determination method applicable to energy storage power stations connected to distribution networks according to claim 8, characterized in that, right After the second data processing, we obtain , is represented as: ； In the formula, for The derivative of .

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