Zero-sequence differential protection method for hybrid power flow controller based on zero-sequence equivalent circuit

Abstract

The invention provides a hybrid power flow controller zero-sequence differential protection method based on a zero-sequence equivalent circuit. The method comprises the steps that the hybrid power flow controller zero-sequence equivalent circuit considering excitation impedance is built according to a hybrid power flow controller topological structure; according to a hybrid power flow controller zero-sequence equivalent circuit, self-produced zero-sequence components of power side current and load side current are calculated respectively, and zero-sequence differential current and zero-sequence braking current are constructed respectively according to the self-produced zero-sequence components; configuring a ratio braking protection criterion according to the zero-sequence differential current and the zero-sequence braking current; on the basis of the zero-sequence differential current, a magnetizing inrush current locking criterion is constructed through Fourier transform; and judging whether zero-sequence differential protection acts or not according to the ratio braking protection criterion and the magnetizing inrush current locking criterion. According to the invention, the reliability and sensitivity of protection are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection for power systems, and in particular to a zero-sequence differential protection method for a hybrid power flow controller. Background Art

[0002] With the large-scale access of random and intermittent renewable energy sources such as wind power and photovoltaic power, the random characteristics of the power flow in China's power grid have become more obvious. The power flow changes greatly and is difficult to control. Some transmission sections are severely blocked, and the utilization rate of some transmission channels is insufficient. The new energy transmission channels are blocked, which seriously restricts the power supply capacity of the power grid and the new energy consumption capacity. A hybrid power flow controller composed of a passive phase-shifting transformer and an active cascaded H-bridge converter can flexibly adjust the line power flow over a large range, thereby improving this situation.

[0003] At present, the relay protection scheme for the existing hybrid power flow controller is not perfect, and it is impossible to accurately distinguish internal and external faults, especially the sensitivity of the action to the turn-to-turn faults of the windings on each side of the series-parallel transformer is insufficient. The current relay protection scheme usually uses Kirchhoff's current law to protect the primary winding of the series-parallel transformer; and protects the secondary winding of the series-parallel transformer according to the ampere-turn balance law. However, this type of scheme uses a large number of current transformer measuring points, which will lead to instability of the scheme, and the misoperation and refusal rate will also be higher.

[0004] In addition, the invention patent with the publication number of CN109066586A discloses a longitudinal zero-sequence differential protection method for a magnetic balance series transformer, which considers the current characteristics of the balance winding when a slight unbalanced fault occurs in the series transformer area, and constructs a differential protection based on the magnetic balance principle; installs current transformers on the network side winding, valve side winding, and balance winding of the series transformer, calculates the self-produced zero-sequence current of each current transformer, and takes the self-produced zero-sequence current of the differential current and the braking current as the calculation quantity, which is not affected by the load current; to a certain extent, it can overcome the braking influence of the load current on the conventional differential protection when reacting to turn-to-turn faults. However, this invention is not applicable to the hybrid power flow controller without a balance winding, and the zero-sequence differential protection constructed by using magnetic balance can react to fewer types of faults and has insufficient reliability. Summary of the Invention

[0005] In view of the technical problem that the sensitivity and reliability of protection actions are reduced when the existing technology faces the complex environment of a hybrid power flow controller, the present invention proposes a zero-sequence differential protection method for a hybrid power flow controller based on a zero-sequence equivalent circuit. Based on the zero-sequence equivalent circuit considering the magnetizing impedances of series and parallel transformers, the zero-sequence equivalent circuits of the hybrid power flow controller after inter-turn faults occur in the windings on each side of the series and parallel transformers are respectively analyzed, as well as the zero-sequence equivalent circuit of the hybrid power flow controller during inrush current. A zero-sequence differential protection for the hybrid power flow controller is constructed, which can effectively reflect the ground asymmetry faults on each side of the primary winding leads of the series and parallel transformers of the hybrid power flow controller and the inter-turn faults in the windings on each side of the series and parallel transformers except for the inter-turn faults of the series transformer, improving the reliability and sensitivity of the protection.

[0006] To achieve the above object, the technical solution of the present invention is realized as follows:

[0007] A zero-sequence differential protection method for a hybrid power flow controller based on a zero-sequence equivalent circuit, comprising the steps of:

[0008] S1: Construct a zero-sequence equivalent circuit of the hybrid power flow controller considering the magnetizing impedance according to the topology of the hybrid power flow controller;

[0009] S2: According to the zero-sequence equivalent circuit of the hybrid power flow controller, calculate the self-produced zero-sequence components of the currents on the power supply side and the load side respectively, and construct a zero-sequence differential current and a zero-sequence braking current according to the self-produced zero-sequence components;

[0010] S3: Configure a ratio braking protection criterion according to the zero-sequence differential current and the zero-sequence braking current;

[0011] S4: Construct an inrush current blocking criterion based on the zero-sequence differential current through Fourier transform;

[0012] S5: Judge whether the zero-sequence differential protection acts according to the ratio braking protection criterion and the inrush current blocking criterion.

[0013] Furthermore, the method for calculating the self-produced zero-sequence components of the currents on the power supply side and the load side respectively is as follows:

[0014] Obtain the three-phase incoming line currents on the power supply side of the three-phase hybrid power flow controller and and the three-phase outgoing line currents on the load side and

[0015]

[0016] wherein, is the zero-sequence component of the current on the power supply side, is the zero-sequence component of the current on the load side.

[0017] Furthermore, the method for constructing the zero-sequence differential current is as follows:

[0018] Take the zero-sequence component of the current on the power supply side and the zero-sequence component of the current on the load side and subtract them to construct the zero-sequence differential current I r_0 , and the calculation formula is:

[0019]

[0020] Furthermore, the method for constructing the zero-sequence braking current is as follows:

[0021] Take the zero-sequence component of the current on the power supply side and the zero-sequence component of the current on the load side and add them to construct the zero-braking current I res_0 ,

[0022] and the calculation formula is:

[0023]

[0024] Furthermore, the method for configuring the ratio-braking protection criterion is as follows:

[0025]

[0026] Among them, I set.min represents the minimum operating current; k is the ratio-braking coefficient; I res.g is the knee-point current, and the minimum operating current I set.min and the knee-point current I res.g are based on the rated value I E of the currents on both sides of the hybrid power flow controller.

[0027] Furthermore, the method for constructing the inrush current blocking criterion is as follows:

[0028] Extract the fundamental wave component amplitude r_0 and the second harmonic component amplitude in the zero-sequence differential current I through Fourier transform FFT to construct the inrush current blocking criterion:

[0029]

[0030] Among them, K d is the second harmonic braking coefficient.

[0031] Further, the method for determining whether the zero-sequence differential protection operates is as follows: if the inrush current blocking criterion is satisfied, it is determined that inrush current occurs, and the operation signal of the ratio restraint protection is blocked; if the ratio restraint protection criterion is satisfied and the inrush current blocking criterion is not satisfied, the zero-sequence differential protection operates.

[0032] Further, when obtaining the three-phase incoming line currents on the power supply side and the three-phase outgoing line currents on the load side of the three-phase hybrid power flow controller, 2 sets of sensors are used.

[0033] Further, the minimum operating current I set.min has a value range of: 0.2 - 0.5I E ; the ratio restraint coefficient takes a value of 0.5 - 0.8; the knee-point current I res.g has a value range of: 0.6 - 1.1I E .

[0034] Further, the second harmonic restraint coefficient K d has a value range of 0.15 - 0.25.

[0035] The beneficial effects of the present invention are as follows:

[0036] The present invention constructs the protection by using the equivalent circuit and operating characteristics of the hybrid power flow controller. This method can effectively reflect the ground asymmetry faults on each side of the series-parallel transformer primary winding leads of the hybrid power flow controller and the turn-to-turn faults in the windings on each side of the series-parallel transformer except for the series transformer turn-to-turn faults, improving the reliability and sensitivity of the protection.

[0037] The present invention does not require complex equipment and a large number of current transformer measuring points, reduces the complexity and instability of the scheme, and improves the reliability of the protection.

[0038] Based on the zero-sequence equivalent circuit and ratio restraint protection, the present invention is not affected by load current, can stably detect faults, and ensures the reliability of the protection.

[0039] With the large-scale access of renewable energy such as wind power and photovoltaic power, the power flow characteristics of the power grid become more complex. The present invention can adapt to this complex operating environment and ensure the safe and stable operation of the hybrid power flow controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0041] Figure 1 It is the topological structure diagram of the hybrid power flow controller.

[0042] Figure 2 It is the A-phase equivalent circuit of the hybrid power flow controller.

[0043] Figure 3 It is the zero-sequence equivalent circuit of the hybrid power flow controller considering the excitation impedance.

[0044] Figure 4 It is the zero-sequence equivalent circuit of the hybrid power flow controller when a series transformer turn-to-turn fault occurs.

[0045] Figure 5 It is the zero-sequence equivalent circuit of the hybrid power flow controller when a parallel transformer turn-to-turn fault occurs.

[0046] Figure 6 It is the operating characteristic of the zero-sequence differential protection.

[0047] Figure 7 It is the operating logic of the zero-sequence differential protection.

[0048] Figure 8 It is the variation of the zero-sequence differential and braking currents when an A-phase asymmetry fault occurs at the midpoint of the series transformer.

[0049] Figure 9 It is the variation of the zero-sequence differential and braking currents when a series transformer turn-to-turn fault occurs.

[0050] Figure 10 It is the variation of the zero-sequence differential and its second harmonic component when inrush current occurs.

[0051] Figure 11 It is the flow chart of the method of the present invention. Specific implementation manners

[0052] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0053] A zero-sequence differential protection method for a hybrid power flow controller based on a zero-sequence equivalent circuit, as Figure 1 shown, includes the steps:

[0054] S1: Establish a zero-sequence equivalent circuit of the hybrid power flow controller considering the excitation impedance according to the topological structure of the hybrid power flow controller.

[0055] Topological Structure Analysis: The hybrid power flow controller consists of a passive phase-shifting transformer and an active cascaded H-bridge converter CHB (the part within the green dashed box). The phase-shifting transformer mainly consists of a series transformer BT (the part within the purple dashed box) and a shunt transformer ET (the part within the red dashed box), as shown in Figure 1 as follows.

[0056] Equivalent Circuit Analysis: According to the Figure 1 topological structure, combined with the equivalent circuit of the transformer, without considering the exciting impedance of the transformer, the windings of each transformer are equivalent to the form of impedance series ideal windings, and the A-phase equivalent circuit is obtained as shown in Figure 2 as follows. Denote the impedances of the windings of the series transformer as Z B1 , Z B2 , Z B3 . Since the primary winding of the series transformer is equally divided and connected in series in the line, i.e., Z B1 = Z B2 , the impedances of the windings of the shunt transformer are Z E1 and Z E2 . The CHB is connected in series at the end of the secondary winding of the shunt transformer, and it can be equivalent to a structure of a controlled voltage source U CHB in series with an impedance Z CHB .

[0057] Zero-Sequence Equivalent Circuit Analysis: Considering the exciting impedances Z BM , Z EM of the series and shunt transformers, the zero-sequence equivalent circuit of the hybrid power flow controller is as shown in Figure 3 as follows, where Z B1 , Z B2 are the impedances of the primary windings of the series transformer, is the equivalent impedance of the secondary winding of the series transformer referred to the primary side, and Z E1 is the impedance of the primary winding of the shunt transformer. During normal operation, the three-phase currents on both sides of the hybrid power flow controller are symmetrical and there is no zero-sequence component (i.e., no action is required), that is, When an asymmetrical grounding fault occurs inside or outside the hybrid power flow controller area, a zero-sequence component will be generated. When an asymmetrical grounding fault occurs outside the hybrid power flow controller area, due to the infinite exciting impedance, the zero-sequence current basically does not flow through the exciting branches of the series and shunt transformers and flows in the direction of the orange dashed line shown in the figure, that is, the zero-sequence currents on both sides of the primary winding of the series transformer are equal (i.e., no action is required). When an asymmetrical grounding fault occurs inside the hybrid power flow controller area, (action is required). Obviously, when there are asymmetrical grounding faults on both sides of the primary windings of the series and shunt transformers, additional zero-sequence branches to the ground will be added to make the protection act.

[0058] Analysis of turn - to - turn faults: For turn - to - turn faults (including inter - turn faults and turn - to - ground faults) in the windings on each side of series - connected and parallel - connected transformers, the zero - sequence equivalent circuit diagrams after the faults are as shown in Figure 4 and 5 . In the figures, Z Bf and Z Ef are the leakage impedances of the fault branches, with very small values similar to the winding impedances. Turn - to - turn faults have the following characteristics:

[0059] The equivalent circuit of the fault branch in an inter - turn fault is connected in parallel on both sides of the exciting branch, as shown by the serial number ① in Figure 5 ; The equivalent circuit of the turn - to - ground fault branch is equivalent to adding a branch to the ground on one side of the exciting branch, as shown by the serial number ② in Figure 4 , Figure 4 and 5 . Faults within the winding group will cause an additional branch to the ground in the zero - sequence equivalent circuit of the controllable phase - shifter, destroying the characteristic of equal zero - sequence currents. Based on this characteristic, the zero - sequence differential protection of the hybrid power flow controller is established in the present invention.

[0060] S2: According to the zero - sequence equivalent circuit of the hybrid power flow controller, calculate the self - generated zero - sequence components of the currents on the power supply side and the load side respectively, and construct the zero - sequence differential current and the zero - sequence braking current based on the self - generated zero - sequence components.

[0061] Obtain the three - phase incoming line currents of the power supply side (SA, SB, and SC) of the three - phase hybrid power flow controller and and and the three - phase outgoing line currents of the load side (LA, LB, and LC) through two groups of sensors and

[0062] Calculate the zero - sequence components of the currents on the power supply side and the load side of the hybrid power flow controller:

[0063]

[0064] Among them, is the zero - sequence component of the current on the power supply side, is the zero - sequence component of the current on the load side.

[0065] Take the difference between the zero - sequence component of the current on the power supply side and the zero - sequence component of the current on the load side to construct the zero - sequence differential current I r_0 , and the calculation formula is:

[0066]

[0067] Add the zero - sequence component of the current on the power supply side and the zero - sequence component of the current on the load side to construct the zero - sequence braking current I res_0 , and the calculation formula is:

[0068]

[0069] S3: Configure the ratio restraint protection criterion according to the zero-sequence differential current and the zero-sequence braking current.

[0070] The operation characteristic diagram is as Figure 6 shown. Obtain the ratio restraint protection criterion according to the operation characteristic diagram:

[0071]

[0072] Among them: I set.min represents the minimum operating current, taking 0.2 - 0.5I E ; k is the ratio restraint coefficient, taking 0.5 - 0.8; I res.g is the knee-point current, taking 0.6 - 1.1I E , and the minimum operating current and the knee-point current are based on the rated value I E of the currents on both sides of the hybrid power flow controller.

[0073] If the ratio restraint protection criterion is satisfied, the ratio restraint protection operates.

[0074] S4: Construct the inrush current blocking criterion based on the zero-sequence differential current through Fourier transform.

[0075] Extract the fundamental component amplitude r_0 in the zero-sequence differential current I and the second harmonic component amplitude

[0076] The inrush current blocking criterion is:

[0077]

[0078] Among them, K d is the second harmonic braking coefficient, taking 0.15 - 0.25.

[0079] S5: Judge whether the zero-sequence differential protection operates according to the ratio restraint protection criterion and the inrush current blocking criterion.

[0080] As Figure 7 shown, if the inrush current blocking criterion is satisfied, it is judged that inrush current occurs and the ratio restraint protection operation signal is blocked; if the ratio restraint protection criterion is satisfied and the inrush current blocking criterion is not satisfied, the zero-sequence differential protection operates.

[0081] Take two typical faults as examples:

[0082] They are respectively: A-phase series transformer neutral point grounding fault; A-phase parallel transformer primary winding 5% turn-to-turn fault.

[0083] In the power system simulation software PSCAD (Power System Computer Aided Design) / EMTDC (Electromagnetic Transients including DC), the changes in the zero-sequence differential current and braking current when the above-mentioned faults occur at 1 s are respectively simulated. As Figure 8 , 9 shown, the blue line in the figure represents the zero-sequence differential current, and the orange line represents the braking current. It can be seen that the zero-sequence differential protection can operate reliably.

[0084] In addition, the changes in the zero-sequence differential current and zero-sequence braking current when inrush current occurs are simulated. As Figure 10 shown, it can be seen that the zero-sequence differential protection can also be reliably blocked in the case of inrush current.

[0085] The above simulations all use the transformer model in PSCAD / EMTDC. The rated parameters of the series transformer and parallel transformer are shown in Table 1 below.

[0086] Table 1 Design parameters of the hybrid power flow controller

[0087]

[0088]

[0089] In addition, for the exciting branches of each transformer (equivalent to a variable exciting impedance, equivalent to infinity during normal operation; sharply reduced to approximately equal to the leakage reactance of the winding during inrush current), the parameters of the exciting branches of each transformer are as follows.

[0090] Parameters of the exciting branch

[0091]

[0092] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A zero-sequence differential protection method for a hybrid power flow controller based on a zero-sequence equivalent circuit, characterized in that: Includes steps: S1: Based on the topological structure of the hybrid power flow controller, a zero-sequence equivalent circuit of the hybrid power flow controller considering the excitation impedance is constructed; S2: According to the zero-sequence equivalent circuit of the hybrid power flow controller, the self-generated zero-sequence components of the current on the power supply side and the load side are calculated respectively, and the zero-sequence differential current and the zero-sequence braking current are constructed respectively according to the self-generated zero-sequence components; S3: Configure the ratio braking protection criterion according to the zero-sequence differential current and the zero-sequence braking current; S4: Construct the magnetizing inrush current blocking criterion based on zero-sequence differential current through Fourier transform; S5: Determine whether the zero-sequence differential protection is activated based on the ratio braking protection criteria and the excitation surge current blocking criteria.

2. The zero-sequence differential protection method of a hybrid power flow controller according to claim 1, characterized in that: The method for calculating the self-generated zero-sequence components of the power supply side and the load side current respectively is: Obtain the three-phase incoming line current on the power supply side of the three-phase hybrid power flow controller and And the three-phase outgoing current on the load side and in, is the zero-sequence component of the current on the power supply side, is the zero-sequence component of the current on the load side.

3. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to claim 2 is characterized in that: The method for constructing zero-sequence differential current is: The zero-sequence component of the current on the power supply side and the zero-sequence current component on the load side Zero sequence differential current I r_0 , the calculation formula is:

4. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to claim 2 is characterized in that: The method for constructing the zero-sequence braking current is: The zero-sequence component of the current on the power supply side and the zero-sequence current component on the load side Adding to construct zero braking current I res_0 , the calculation formula is:

5. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to claim 4 is characterized in that: The method for configuring the ratio braking protection criterion is: Among them, I set.min Indicates the minimum operating current; k is the ratio braking coefficient; I res.g is the inflection point current, the minimum operating current I set.min and the inflection point current I res.g The rated value of the current on both sides of the hybrid power flow controller is I E As a benchmark.

6. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to claim 5 is characterized in that: The method for constructing the excitation inrush current blocking criterion is: Extract zero-sequence differential current I by Fourier transform FFT r_0 Fundamental component amplitude And the amplitude of the second harmonic component Construct excitation inrush current blocking criteria: Among them, K d is the second harmonic braking coefficient.

7. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to any one of claims 3 to 6, characterized in that: The method for judging whether the zero-sequence differential protection is actuated is as follows: if the excitation surge current blocking criterion is met, it is judged that an excitation surge current occurs, and the ratio braking protection action signal is blocked; if the ratio braking protection criterion is met and the excitation surge current blocking criterion is not met, the zero-sequence differential protection is actuated.

8. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to any one of claims 2 to 4, characterized in that: Two sets of sensors are used to obtain the three-phase incoming current on the power supply side and the three-phase outgoing current on the load side of the three-phase hybrid power flow controller.

9. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to claim 5 or 6, characterized in that: The minimum operating current I set.min The value range is: 0.2~0.5I E ; The ratio braking coefficient is 0.5~0.8; The inflection point current I res.g The value range is: 0.6~1.1I E .

10. The zero-sequence differential protection method of a hybrid power flow controller based on a zero-sequence equivalent circuit according to claim 6, characterized in that: The second harmonic braking coefficient K d The value range is 0.15 to 0.25.

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