Protection system

Abstract

To cope with a single phase grounding accident in a three-phase AC power system.SOLUTION: A protection system (110) for protecting a three-phase AC power system (100) comprises a protection unit (12) connected to a neutral point of the power system (100). The neutral point is grounded through the protection unit (12). The protection unit (12) comprises a neutral compensation reactor (10) and a neutral point capacitor (for example: capacitor 6) connected in series with the neutral compensation reactor (10).SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present disclosure relates to a protection system for protecting a three-phase AC power system.

Background Art

[0002] Various techniques for protecting a three-phase AC power system have been proposed. As an example, Patent Document 1 below discloses a technique aimed at dealing with a three-phase short-circuit accident or a three-phase ground-fault accident.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of one aspect of the present disclosure is to deal with a single-phase ground-fault accident in a three-phase AC power system.

Means for Solving the Problems

[0005] A protection system according to one aspect of the present disclosure is a protection system for protecting a three-phase AC power system, comprising a protection unit connected to a neutral point in the power system, the neutral point being grounded via the protection unit, and the protection unit comprising a neutral-point compensation reactor and a neutral-point capacitor connected in series with the neutral-point compensation reactor.

Effects of the Invention

[0006] According to one aspect of the present disclosure, a single-phase ground-fault accident in a three-phase AC power system can be dealt with.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Mode for Carrying Out the Invention

[0008] 〔Reference Form〕 Prior to the description of Embodiment 1, the reference form will be described. For convenience of explanation, components having the same functions as the components (components) described in the reference form will be given the same reference numerals in the following embodiments, and the description thereof will not be repeated. Also, for the sake of brevity, descriptions of matters similar to known techniques will be omitted as appropriate.

[0009] Each component and each numerical value described in this specification are all merely illustrative examples unless there is a particular contradiction. Therefore, for example, unless there is a particular contradiction, the positional relationship and connection relationship of each component are not limited to the examples in each figure. The description "connected" in this specification means "electrically connected" unless there is no contradiction in content.

[0010] As described above, the technology of Patent Document 1 aims to deal with three-phase short-circuit accidents or three-phase ground-fault accidents in the power system. However, in Patent Document 1, single-phase ground-fault accidents in the power system are not considered. Therefore, in the prior art, there may be cases where single-phase ground-fault accidents in the power system cannot be appropriately addressed. This will be described in the reference embodiment.

[0011] FIG. 1 shows a configuration example of a three-phase AC power system 120R in the reference embodiment. The power system 120R is an example of the prior art. In FIG. 1, the three-phase circuit is indicated by a thick line, and the single-phase circuit is indicated by a thin line.

[0012] The power system 120R has a three-phase AC power source 1. As an example, the power source 1 may be any renewable energy power generation facility (e.g., a wind power generation facility). The power source 1 is connected to another power generation facility (not shown) via a transmission cable 8. Therefore, the power source 1 can be connected to the other power generation facility via the connection point P2 shown in FIG. 1.

[0013] Reference numeral 2 in FIG. 1 indicates the impedance behind the power source 1. The power system 120R has a circuit breaker 4 between the power source 1 and the transmission cable 8. The circuit breaker 4 in the example of FIG. 1 is arranged to cut off the fault current at the connection point P2. In FIG. 1, a case where a single-phase ground-fault accident occurs at a fault point P1 located closer to the power source 1 than the connection point P2 is illustrated.

[0014] When a single-phase ground-fault accident occurs at the fault point P1, current flows from the transmission cable 8 towards the circuit breaker 4. The current i in FIG. 1 CABLE represents the current flowing from the transmission cable 8 towards the circuit breaker 4.

[0015] In the example of FIG. 1, the power supply 1 is connected in parallel to the power transmission cable 8 and the electrical equipment 13R via the circuit breaker 4. The electrical equipment 13R is, for example, power receiving equipment that receives power supply from the power supply 1. The power system 120R may have a transformer 11 that converts the voltage of the power supply 1. In the example of FIG. 1, the transformer 11 is located inside the electrical equipment 13R. The transformer 11 may be any type of transformer as long as it is a three-phase transformer. In FIG. 1, a three-phase three-winding transformer is illustrated as the transformer 11.

[0016] In the example of FIG. 1, the primary winding of the transformer 11 is Y-connected, and the secondary winding of the transformer 11 is Δ-connected. The primary side of the transformer 11 is connected to the power supply 1. The secondary side of the transformer 11 is connected to a load (not shown) inside the electrical equipment 13R.

[0017] Depending on the specifications of the power system, a fairly long power transmission cable may be used. For this reason, the power transmission cable may have a relatively large capacitance. Therefore, in the power system, a neutral point compensation reactor 10 may be provided to compensate for the capacitance of the power transmission cable. The neutral point compensation reactor may be abbreviated as NGL.

[0018] As shown in FIG. 1, the power system 120R has a neutral point compensation reactor 10 inside the electrical equipment 13R. FIG. 2 shows an example of the connection relationship of the neutral point compensation reactor 10 in a reference configuration. In FIG. 2, the connection relationship between the neutral point compensation reactor 10 and the transformer 11 is mainly illustrated.

[0019] The neutral point compensation reactor 10 may be any type of reactor. As an example, an air-core reactor may be used as the neutral point compensation reactor 10. The reactance of the neutral point compensation reactor 10 is set to correspond to the capacitance of the power transmission cable 8. Therefore, the reactance of the neutral point compensation reactor 10 is relatively large.

[0020] It is assumed that the power system 120R has a neutral point NP of a three-phase circuit. In the example of FIG. 2, the neutral point NP is located at the junction of the Y connection of the transformer 11. The voltage V0 in FIG. 2 indicates the voltage of the neutral point NP. The voltage V0 is also referred to as the neutral point voltage. As is known to those skilled in the art, the neutral point voltage is equal to the zero-phase voltage in the power system 120R.

[0021] As shown in FIG. 2, one end of the neutral point compensation reactor 10 is connected to the neutral point NP. The other end of the neutral point compensation reactor 10 is grounded. In the example of FIG. 2, the grounding terminal connected to the other end of the neutral point compensation reactor 10 is indicated by the symbol GN.

[0022] When a single-phase-to-ground fault occurs at the fault point P1 of the power system 120R, a current flows from the neutral point compensation reactor 10 towards the circuit breaker 4. The current i in FIG. 2 n represents the current flowing from the neutral point compensation reactor 10 towards the circuit breaker 4.

[0023] FIG. 3 schematically shows each current that occurs when a single-phase-to-ground fault occurs in the power system 120R. In each graph of FIG. 3, the horizontal axis represents the time point t, and the vertical axis represents the current value. The initial time point t = 0 in the example of FIG. 3 indicates the time point when the fault occurs.

[0024] The symbol 310 in FIG. 3 indicates an example of the time variation of the current i CABLE . In the example of FIG. 3, i CABLE represents the zero-phase current. The actual i CABLE includes a positive-phase current component in addition to the zero-phase current component, but in the example of FIG. 3, the positive-phase current component is not considered. In the example of FIG. 3, it is assumed that the positive-phase current component is compensated by a shunt reactor (not shown) for positive-phase compensation separately installed in the power system 120R.

[0025] The i in FIG. 3 CABLE is an alternating current that does not include a direct current component. The frequency of i CABLE is equal to the rated frequency (fundamental wave frequency) of the power system 120R. In the example of FIG. 3, at t = 0, iCABLE shall take the maximum value.

[0026] Reference numeral 320 in FIG. 3 indicates an example of the time variation of the current i n When a single-phase-to-ground fault occurs in the power system 120R, an imbalance occurs in the three-phase AC voltage in the power system 120R. Therefore, when a single-phase-to-ground fault occurs in the power system 120R, a non-zero V0 (e.g., positive V0) occurs.

[0027] Due to the non-zero V0 being applied to the neutral-point compensation reactor 10, i n will include a DC component i DC In the example of reference numeral 320, i DC is a positive signal that monotonically decreases with time. The value of i DC at t = 0 is determined according to the occurrence timing of the ground fault (more specifically, according to the phase of the voltage to ground at the time of the ground fault).

[0028] Also, in the power system 120R, i n will include an AC component that is out of phase with i CABLE Since the neutral-point compensation reactor 10 is provided to compensate for the current in the transmission cable 8, in the example of reference numeral 320, i n is represented by the following formula (1), i n = -i CABLE + i DC …(1) and is represented by

[0029] In this specification, the current flowing through the circuit breaker 4 is denoted as i CB Reference numeral 330 in FIG. 3 indicates an example of the time variation of i CB According to the circuit configuration of FIG. 1, when a single-phase-to-ground fault occurs in the power system 120R, i CB is represented by the following formula (2), i CB = i n + i CABLE …(2) and is represented by

[0030] Then, by substituting Equation (1) into Equation (2), the following Equation (3) is obtained: i CB = (-i CABLE + i DC ) + i CABLE = i DC …(3) is obtained.

[0031] That is, in the power system 120R, when a single-phase ground fault occurs, i CB is equal to i DC . The graph with reference numeral 330 illustrates this fact.

[0032] As shown in FIG. 1, in the power system 120R, i DC flows through the grounding terminal GN to the fault point P1 and returns to the grounding terminal GN through the connection point P2. Thus, in the power system 120R, when a single-phase ground fault occurs, there is a risk that a direct current will continue to flow through the circuit breaker 4 for a long time. That is, in the power system 120R, when a single-phase ground fault occurs, there is a risk that the phenomenon that the zero-crossing point of i CB does not occur for a long time will occur. This phenomenon is referred to as a current zero miss.

[0033] The technique disclosed in Patent Document 1 described above aims to prevent a current zero miss when a three-phase short circuit fault or a three-phase ground fault occurs. However, in Patent Document 1, a single-phase ground fault is not considered. Thus, in the conventional technique, the countermeasures against a single-phase ground fault are not sufficient.

[0034] [Embodiment 1] Based on the above problems in the conventional technique, the inventors of the present application (hereinafter abbreviated as "the inventors") newly created Embodiment 1. FIG. 4 shows a configuration example of the power system 100 in Embodiment 1. FIG. 4 is a diagram paired with FIG. 1. The power system 100 has a protection system 110 that protects the power system 100. In this regard, the power system 100 is different from the power system 120R in the reference embodiment.

[0035] In the power system 100 of FIG. 4, the circuit breaker 4 may be a component of the protection system 110. The circuit breaker 4 may be located upstream of the protection unit 12 described below. Also in the example of FIG. 4, the circuit breaker 4 is located at the connection point P2 in the power system 100. The protection system 110 having the circuit breaker 4 may be referred to as a circuit breaker system.

[0036] The protection system 110 has a protection unit 12 instead of the neutral point compensation reactor 10 in the power system 120R described above. For convenience of explanation, the electrical equipment in Embodiment 1 is referred to as electrical equipment 13. FIG. 5 shows an example of the connection relationship of the protection unit 12 in Embodiment 1. In FIG. 5, the connection relationship between the protection unit 12 and the transformer 11 is mainly illustrated. FIG. 5 is a figure paired with FIG. 2.

[0037] The transformer 11 in Embodiment 1 is located between the connection point P2 and the protection unit 12. Also in Embodiment 1, the neutral point NP is located at the junction of the Y connection of the transformer 11. However, the position of the neutral point NP is not necessarily limited to this example.

[0038] For example, the transformer 11 may be a grounding transformer. In this case, the neutral point NP may be located at the junction of the zigzag winding of the grounding transformer.

[0039] As shown in FIG. 5, the neutral point NP may be grounded via the protection unit 12. That is, the neutral point NP may be connected to the grounding terminal GN via the protection unit 12.

[0040] The protection unit 12 has the neutral point compensation reactor 10 described above. And the protection unit 12 further has a capacitor 6. The capacitor 6 is a component associated with the neutral point compensation reactor 10. From this, the capacitor 6 may be referred to as a neutral point capacitor.

[0041] As described below, the capacitor 6 is provided for the purpose of preventing current zero errors in the power system 100. Therefore, the capacitor 6 may be referred to as, for example, a "zero-crossing capacitor". The capacitor 6 is a novel component not assumed in the prior art.

[0042] In the protection unit 12, the capacitor 6 is connected in series with the neutral point compensation reactor 10. In FIG. 5, a configuration in which the neutral point compensation reactor 10 is connected to the neutral point NP and the capacitor 6 is connected to the grounding terminal GN is illustrated. However, the positional relationship between the neutral point compensation reactor 10 and the capacitor 6 is not limited to this example. For example, in the protection unit 12, the capacitor 6 may be connected to the neutral point NP and the neutral point compensation reactor 10 may be connected to the grounding terminal GN.

[0043] i in Embodiment 1 n represents the current flowing from the protection unit 12 towards the circuit breaker 4. As described above, in the protection unit 12, the neutral point compensation reactor 10 and the capacitor 6 are connected in series. Therefore, i in Embodiment 1 n is equal to the current flowing through each of the neutral point compensation reactor 10 and the capacitor 6.

[0044] In the protection unit 12, an electrical resonance phenomenon can be caused by the neutral point compensation reactor 10 and the capacitor 6 connected in series with each other. Therefore, for example, according to the protection unit 12, a resonance current can be generated when a single-phase ground fault occurs in the power system 100.

[0045] As described above, when a single-phase ground fault occurs in the power system 100, a non-zero zero-phase current V0 may occur. The protection unit 12 generates a resonance current i RESO corresponding to the zero-phase current V0. Therefore, when a single-phase ground fault occurs in the power system 100, the i n generated may include i RESO . i RESO is in may also be referred to as a resonance component.

[0046] (i RESO description) Hereinafter, i RESO and related elements will be mathematically examined. In Embodiment 1, consider the case where i RESO takes its maximum value at t = 0. In this case, i RESO is given by the following equation (4),

Equation

[0047] In Equation (4), V L is the peak value of the voltage applied to the protection unit 12 (i.e., the voltage V0 of the neutral point NP). L is the inductance of the neutral point compensation reactor 10. C is the capacitance of the capacitor 6. ω0 is the rated angular frequency in the power system 100. ω0 is given as ω0 = 2πf0. f0 is the rated frequency in the power system 100.

[0048] In Embodiment 1, the reactance of the protection unit 12 is denoted as X. X is given by the following equation (5),

Equation

[0049] Alternatively, X can be represented by the following equation (6),

Equation

[0050] By substituting Equation (5) into Equation (4), the following equation (7),

Equation

[0051] FIG. 6 schematically shows each current that occurs when a single-phase-to-ground fault occurs in the power system 100. FIG. 6 is a figure paired with FIG. 3. In FIG. 6, reference numeral 610 shows an example of the time variation of i CABLE and reference numeral 620 shows an example of the time variation of i n and reference numeral 630 shows an example of the time variation of i CB . In FIG. 6, i CABLE is equivalent to i CABLE in FIG. 3.

[0052] In Embodiment 1, i n includes i DC instead of i RESO in the reference embodiment. Therefore, in the example of reference numeral 620, i n is expressed by the following formula (8), i n =-i CABLE +i RESO …(8) and is represented as such.

[0053] In Embodiment 1, i CB is also represented by the above formula (2). Therefore, by substituting formula (8) into formula (2), the following formula (9), i CB =(-i CABLE +i RESO )+i CABLE =i RESO …(9) is obtained.

[0054] That is, in the power system 100, i CB when a single-phase-to-ground fault occurs is equal to i RESO . The graph of reference numeral 630 illustrates this. As described above, i CB in Embodiment 1 does not include i CB and is different from i DC in the reference embodiment.

[0055] Therefore, according to Embodiment 1, compared with the reference embodiment, the zero-crossing point of i CB can be generated earlier. As can be understood from FIG. 6, the shorter the period of i RESO , the earlier the zero-crossing point of i CB can be generated. When i CB does not include i DC , the zero-crossing point of i CB exists every half cycle of i RESO .

[0056] In Embodiment 1, if there is a zero-crossing point of i CB within a predetermined time T from a certain point in time (e.g., the opening operation time of the circuit breaker 4), it is assumed that the circuit breaker 4 can be normally turned off. T in Embodiment 1 is given as the length of the current zero error evaluation time of the circuit breaker 4. Examples of the current zero error evaluation time will be described later.

[0057] In Embodiment 1, if T is longer than the half cycle of i RESO , when a single-phase ground fault occurs, the circuit breaker 4 can be surely turned off. Therefore, in Embodiment 1, the following formula (10),

Equation

[0058] Here, by solving the above (5) for L, the following formula (11),

Equation

[0059] Then, by substituting formula (11) into formula (10), the following formula (12),

Equation

[0060] By solving Equation (12) for C, the following Equation (13),

Number

[0061] The right side in Equation (13) may theoretically take a negative value depending on the values of T and ω0. However, in Embodiment 1, it is assumed that C is positive in order to make the protection unit 12 function as a compensation reactor for the fundamental wave component of the current.

[0062] Therefore, in Embodiment 1, the following Equation (14),

Number

[0063] (Example of simulation result) The inventors performed a simulation to examine the effectiveness of the protection system 110. Specifically, the inventors simulated a single-phase to ground fault in the power system 100. The inventors simulated the case where a ground fault occurred in the U phase among the three phases (U phase, V phase, and W phase) in the power system 100.

[0064] The simulation conditions were · Rated voltage of the power system 100: 154 kV · Rated frequency (f0) of the power system 100: 50 Hz · Capacitance of the transmission cable 8: 100 Mvar <Case of only neutral point compensation reactor 10: the case of FIG. 7 described below> · Capacity of neutral point compensation reactor 10: 100 MVA <Case of providing protection unit 12: the case of FIG. 8 described below> · Capacity of neutral point compensation reactor 10: 105 MVA · Capacity of capacitor 6: 5 Mvar · Capacity of protection unit 12 (Q L1 ): 100 Mvar is as follows.

[0065] First, as a comparative example, the inventors performed a simulation for the case where C that satisfies the above formula (14) is not provided. Specifically, as a comparative example, the inventors performed a simulation for the case where capacitor 6 is not provided. The comparative example corresponds to the above reference embodiment.

[0066] FIG. 7 shows the simulation results in the comparative example. In the following description, the voltage at connection point P2 is denoted as V p2 . V p2 is also referred to as the connection point voltage. Reference numeral 710 in FIG. 7 indicates V p2 in the comparative example. In the simulation, V p2 for three phases is derived. The accident occurrence time in the simulation is set to t0 = 0.195 s (seconds). Therefore, in the graph of reference numeral 710, after t0, the U-phase voltage of V p2 is maintained at 0 V.

[0067] Reference numeral 720 in FIG. 7 indicates i CB in the comparative example. In Embodiment 1, it is assumed that the opening operation of circuit breaker 4 (operation to physically open the contacts of circuit breaker 4) is performed at a time about 30 ms after the accident occurrence time. Therefore, in the simulation, the time t1 at 0.030 s (i.e., 30 ms) after the accident occurrence time is set as the opening operation time of circuit breaker 4. t1 = 0.195 s + 0.030 s = 0.225 s.

[0068] In the circuit breaker 4 of the first embodiment, a time range of about 60 ms from the time of the contact opening operation is assumed as the zero current miss evaluation time. Therefore, in the simulation, T is set to 60 ms. In the simulation, t1 is set as the start point of the zero current miss evaluation time. Then, time te, which is 0.060 s (i.e., 60 ms) after t1, is set as the end point of the zero current miss evaluation time.

[0069] Therefore, te in the simulation can be said to be the zero-miss evaluation time, including the relay operation time and opening operation time of the circuit breaker 4 from the start of the simulation. In the example of Figure 7, te = t1 + T = 0.225 s + 0.060 ms = 0.285 s. In this way, te in the simulation is set as a point 0.090 s (i.e., 90 ms) after t1.

[0070] As shown in the graph of reference numeral 720, in the comparative example, i CB In the comparative example, even if a long time has passed since t0 (for example, even if 200 ms or more have passed since t0), the zero crossing point of i CB In this way, the zero crossing point of i CB corresponds to the example of code 330 above.

[0071] As described above, the simulation showed that if capacitor 6 is not provided, it will not be possible to adequately deal with a current zero miss when a one-phase ground fault occurs in power system 100 (e.g., the fault current will not be interrupted by circuit breaker 4 within the current zero miss evaluation time).

[0072] Next, the inventors performed a simulation as an example in which C satisfying the above-mentioned formula (14) was provided. The example corresponds to embodiment 1. By applying the above-mentioned simulation conditions to formula (14), the following formula (15):

number

[0073] Then, by calculating the right side of Equation (15), the following Equation (16), 0 μF < C < 1410 μF …(16) is obtained.

[0074] Therefore, in the embodiment, it is only necessary that C is set so that Equation (16) is satisfied. As an example, the inventors set C to C = 765 μF in the embodiment. This value of C is selected so as to satisfy the relationship of X C / X L = 5%. X C is the reactance of capacitor 6, and X L is the reactance of neutral point compensation reactor 10.

[0075] Figure 8 shows the simulation results in the embodiment. Figure 8 is a figure paired with Figure 7. Reference numeral 810 in Figure 8 indicates V p2 in the embodiment. The graph of reference numeral 810 is equivalent to the graph of reference numeral 710.

[0076] Reference numeral 820 in Figure 8 indicates i CB in the embodiment. I CB in the embodiment corresponds to the example of reference numeral 630 described above. As shown in the graph of reference numeral 820, in the embodiment, there is a zero crossing point of i CB within the current zero miss evaluation time. In the embodiment, the time point tzc 70 ms after t0 is the zero crossing point of i CB .

[0077] Thus, in the embodiment, by providing capacitor 6 that satisfies the above-described Equation (14), the zero crossing point of i CB can be generated earlier than in the comparative example. As described above, the effectiveness of combining neutral point compensation reactor 10 and capacitor 6 (that is, the effectiveness of protection unit 12) was confirmed through simulation by the inventors.

[0078] (Effect) As described above, according to the protection unit 12, different from the prior art, it is possible to cope with a single-phase ground fault in a three-phase AC power system. Specifically, it can effectively cope with the current zero error in the case of a single-phase ground fault in the power system. As a result, for example, when a single-phase ground fault occurs in the power system, the fault current can be quickly interrupted by the circuit breaker 4.

[0079] Also, as shown in FIG. 4, according to the first embodiment, by providing only one protection unit 12, that is, a protection unit 12 for one phase, the above-described respective effects can be obtained. Therefore, by simply adding a configuration to the existing equipment, it is possible to effectively cope with a single-phase ground fault. Therefore, the method for coping with the current zero error in the first embodiment is also beneficial in terms of cost.

[0080] Also, as described above, in the first embodiment, an air-core reactor is used as the neutral-point compensation reactor 10. In an air-core reactor, different from an iron-core reactor, fractional harmonic oscillation in the reactor does not occur. Therefore, when an air-core reactor is used as the neutral-point compensation reactor 10, overvoltage and overcurrent caused by the fractional harmonic oscillation do not occur in the neutral-point compensation reactor 10.

[0081] (Supplement in the First Embodiment) (1) Different from the example in FIG. 4, the protection system 110 may have a plurality of protection units 12 connected in parallel with each other. As an example, the protection system 110 may have three protection units 12. The protection system 110 in the first embodiment only needs to have the same number of capacitors 6 as the neutral-point compensation reactor 10.

[0082] (2) In the first embodiment, the case where a single-phase ground fault occurs in the power system 100 is illustrated. However, it should be noted that the protection system 110 can also cope with accidents other than single-phase ground faults. For example, the protection system 110 can also cope with two-phase ground faults.

[0083] [Second Embodiment] In the protection system according to one aspect of the present disclosure, the number of neutral point compensation reactors and the number of capacitors do not have to be the same. In Embodiment 2, an example of a protection system in which the number of neutral point compensation reactors and the number of capacitors are different will be described.

[0084] FIG. 9 shows a configuration example of the power system 100 in Embodiment 2. FIG. 9 is a diagram paired with FIG. 4. As shown in FIG. 9, the power system 100 in Embodiment 2 has a protection system 110A instead of the protection system 110.

[0085] The protection system 110A has an additional neutral point compensation reactor 18A, which is different from the neutral point compensation reactor 10 of the protection unit 12. For convenience of explanation, the neutral point compensation reactor 10 may be referred to as the first neutral point compensation reactor, and the neutral point compensation reactor 18A may be referred to as the second neutral point compensation reactor. Thus, the protection system 110A has two neutral point compensation reactors and one capacitor.

[0086] As shown in FIG. 9, the neutral point compensation reactor 18A is located outside the protection unit 12. And the neutral point compensation reactor 18A is connected in parallel with the protection unit 12. Therefore, the neutral point compensation reactor 18A is connected to the neutral point NP and grounded.

[0087] In the example of FIG. 9, the grounding terminal corresponding to the protection unit 12 is represented by the symbol GN1, and the grounding terminal corresponding to the neutral point compensation reactor 18A is represented by the symbol GN2. For convenience of explanation, the grounding terminal GN1 may be referred to as the first grounding terminal, and the grounding terminal GN2 may be referred to as the second grounding terminal. The grounding terminal GN1 corresponds to the grounding terminal GN in Embodiment 1. The protection unit 12 is connected to the grounding terminal GN1. The neutral point compensation reactor 18A is connected to the grounding terminal GN2.

[0088] As shown in FIG. 9, in Embodiment 2, the current flowing from the protection unit 12 toward the circuit breaker 4 is i n1is denoted as such. On the other hand, the current flowing from the neutral point compensation reactor 18A towards the circuit breaker 4 is denoted as i n2 is denoted as such.

[0089] FIG. 10 schematically shows each current that occurs when a single-phase-to-ground fault occurs in the power system 100 of Embodiment 2. FIG. 10 is a figure paired with FIG. 6. In FIG. 10, reference numeral 1010 shows an example of the time variation of i CABLE , reference numeral 1020 shows an example of the time variation of i n2 , reference numeral 1030 shows an example of the time variation of i n1 , and reference numeral 1040 shows an example of the time variation of i CB . The i CABLE in FIG. 10 is equivalent to the i CABLE in FIG. 6.

[0090] As described above, the neutral point compensation reactor 18A is not connected to the capacitor 6. Therefore, the i n2 in Embodiment 2 corresponds to the i n in the reference embodiment. From this, the i n2 in the example of reference numeral 1120 is given by the following formula (17), i<( n2 =-α×i CABLE +i DC …(17) where α is a coefficient determined by the ratio of the impedance of the protection unit 12 to the impedance of the neutral point compensation reactor 18A.

[0091] On the other hand, the i n1 in Embodiment 2 corresponds to the i n in the embodiment. From this, the i n1 in the example of reference numeral 1130 is given by the following formula (18), i n1 =-(1-α)×i CABLE +i RESO …(18) is represented as such.

[0092] According to the circuit configuration of FIG. 9, in the case where a single-phase-to-ground fault occurs in the power system 100 of Embodiment 2, the iCB is represented by the following formula (19), i CB = i n1 + i n2 + i CABLE …(19) as follows.

[0093] Therefore, by substituting formulas (17) and (18) into formula (19), the following formula (20), i CB = i RESO + i DC …(20) is obtained.

[0094] As described above, in the power system 100 of Embodiment 2, when a single-phase ground fault occurs, i CB has a waveform in which i RESO is superimposed on i DC . The graph of reference numeral 1040 illustrates this.

[0095] As described above, in Embodiment 2, unlike Embodiment 1, i CB includes i DC . However, also in Embodiment 2, as in Embodiment 1, i CB includes i RESO . Therefore, by appropriately designing the protection system 110A, it becomes possible to cope with a single-phase ground fault.

[0096] As an example, if the amplitude of i RESO in formula (20) is larger than the maximum value of i DC in the same formula, there will be a zero crossing point in i CB of Embodiment 2. Therefore, in Embodiment 2, the protection system 110A is configured such that i n1 is larger than i n2 .

[0097] In Embodiment 2, the capacitance of the neutral point compensation reactor 18A is denoted as Q L2 . In Embodiment 2, the capacitance Q L1 of the protection unit 12 is QL2 The protection system 110A is configured to be larger than

[0098] That is, in Embodiment 2, the following formula (21): Q L1 >Q L2 …(21) is satisfied.

[0099] By configuring the protection system 110A to satisfy the formula (21), i n1 can be made larger than i n2 . Therefore, the zero-crossing point of i in Embodiment 2 CB can be made to occur more reliably.

[0100] As described above, i in Embodiment 2 CB includes i DC . Therefore, in Embodiment 2, there may be cases where there is no zero-crossing point of i RESO for each half cycle of i CB . However, for example, if the above formula (21) is satisfied, the zero-crossing point of i RESO is guaranteed to exist for each cycle of i CB .

[0101] Therefore, in Embodiment 2, instead of the above formula (12), the following formula (22):

Number

[0102] Based on the formula (22), by deriving the conditional formula that C should satisfy in Embodiment 2 according to the same concept as in Embodiment 1, the following formula (23):

Number

[0103] Therefore, in Embodiment 2, instead of the above-described formula (14) in Embodiment 1, the protection system 110A may be configured such that formula (23) is satisfied. According to the protection system 110A that satisfies formula (23), as with the protection system 110 of Embodiment 1, it is possible to effectively handle a single-phase ground fault.

[0104] 〔Summary〕 The protection system according to Aspect 1 of the present disclosure is a protection system that protects a three-phase AC power system, and includes a protection unit connected to the neutral point in the power system. The neutral point is grounded via the protection unit, and the protection unit includes a neutral point compensation reactor and a neutral point capacitor connected in series with the neutral point compensation reactor.

[0105] The protection system according to Aspect 2 of the present disclosure may include a circuit breaker located upstream of the protection unit in Aspect 1, and the circuit breaker may be located at a connection point in the power system.

[0106] In the protection system according to Aspect 3 of the present disclosure, in Aspect 2, when the rated angular frequency in the power system is denoted as ω0, the reactance of the protection unit is denoted as X, the length of the current zero miss evaluation time of the circuit breaker is denoted as T, and the capacitance of the neutral point capacitor is denoted as C, the above-described formula (14) may be satisfied.

[0107] The protection system according to Aspect 4 of the present disclosure may include an additional neutral point compensation reactor connected in parallel with the protection unit in Aspect 2. The additional neutral point compensation reactor is connected to the neutral point and may be grounded. When the rated angular frequency in the power system is denoted as ω0, the reactance of the protection unit is denoted as X, the length of the current zero miss evaluation time of the circuit breaker is denoted as T, and the capacitance of the neutral point capacitor is denoted as C, the above-described formula (23) may be satisfied.

[0108] In the protection system according to Embodiment 5 of the present disclosure, in Embodiment 4, the capacity of the protection unit may be greater than the capacity of the additional neutral point compensation reactor.

[0109] In the protection system according to Embodiment 6 of the present disclosure, in any one of Embodiments 1 to 5, the protection unit may generate a resonance current when a single-phase ground fault occurs in the power system.

[0110] In the protection system according to Embodiment 7 of the present disclosure, in any one of Embodiments 1 to 6, in the power system, a three-phase transformer may be located between the connection point in the power system and the protection unit, and the neutral point may be located at the junction point of the Y connection of the transformer.

[0111] In the protection system according to Embodiment 8 of the present disclosure, in any one of Embodiments 1 to 6, in the power system, a three-phase transformer may be located between the connection point in the power system and the protection unit, the transformer may be a grounding transformer, and the neutral point may be located at the junction point of the zigzag winding of the grounding transformer.

[0112] 〔Supplementary Notes〕 The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure.

Explanation of Reference Numerals

[0113] 100 Power system 110 Protection system 4 Circuit breaker 6 Capacitor (neutral point capacitor) 10 Neutral point compensation reactor (neutral point compensation reactor connected in series with the neutral point capacitor) 11 Transformer 12 Protection unit 18A Neutral point compensation reactor (additional neutral point compensation reactor) 110A protection system (protection system with additional neutral point compensation reactor) NP neutral point P2 connection point i RESO resonance current

Claims

1. A protection system for protecting a three-phase AC power system, comprising: a protection unit connected to the neutral point in the power system, wherein the neutral point is grounded via the protection unit, and the protection unit comprises: a neutral point compensation reactor, and a neutral point capacitor connected in series with the neutral point compensation reactor.

2. The protection system further comprises a circuit breaker located upstream of the protection unit, wherein the circuit breaker is located at the interconnection point in the power system. The protection system according to claim 1.

3. Let the rated angular frequency in the power system be ω ０ and denote it as When the reactance of the protection unit is denoted as X, the length of the current zero miss evaluation time of the circuit breaker is denoted as T, and the capacitance of the neutral point capacitor is denoted as C, [[ID=1 【Number 1】 …（１） ​ ​ ​ ​ Let the rated angular frequency in the power system be ω ０ and denote it as ​ ​ ​ ​ 【Number 2】 …（２） ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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