Small current grounding systems are a major component of rural power grids, and grounding faults, ferroresonance, PT disconnection, and line disconnection are common faults in small current grounding power grids, requiring manual troubleshooting. When these faults occur, they share a common characteristic: the generation of a grounding signal (specifically for single-source, single-circuit transmission lines). Grounding and resonance are discussed in detail in some books and regulations, and are relatively familiar to most users. However, when generating a grounding signal, some operators easily overlook whether the PT circuit is normal, especially lacking understanding of the characteristics of a transmission line disconnection, often misjudging it as a grounding fault, causing unnecessary grounding-selective power outages and delaying accident handling. Therefore, it is necessary to calculate and analyze the latter two types of faults and compare the characteristics of each fault. [b]1 Voltage Calculation and Analysis During Faults[/b] 1.1 Voltage calculation and analysis during PT fault. Under normal conditions, since 3U0 is taken from the PT's turns ratio [img=38,44]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/g1-52.gif[/img]／[img=38,41]http://zszl.cepee.com/cepee_kjlw_pic/fil Therefore, the voltage Ua = Ub = Uc = 100/3 V for the three windings of the PT open delta connection. (Image URL: es/wx/hubeidl/hbdl99/hbdl9904/image/g2-52.gif) (1) When the open delta winding is connected in reverse to one phase (c phase), 3[img=146,24]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/g5-52.gif[/img]＝－2[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] c [/sub], that is, 3U[sub] 0 [/sub]＝66.7V; When the two phases (b, c) are reversed, 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] a [/sub]－[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] b [/sub]－[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] c [/sub]＝2[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] a [/sub]，that is, 3U[sub] 0 [/sub]＝66.7V。 (2) Secondary neutral line disconnection When the secondary neutral line is disconnected, since the secondary load of each phase is the same, the secondary three-phase voltage remains unchanged, and the indication is Ua＝Ub＝Uc＝100／＝57.7V; when a single-phase grounding occurs in the primary system, since the voltage triangle Δabc formed by the secondary three-phase voltage is an equilateral triangle, the three-phase symmetrical voltage generated by the same secondary load of each phase forms a break voltage of 57.7V at the secondary neutral line break, so the secondary three-phase voltage remains unchanged, and the indication is 57.7V, but the open delta voltage is 100V. (3) One-phase (two-phase) disconnection: Since there is a load between the secondary phases of the PT and each phase, the circuit formed by the load impedance determines the phase disconnection voltage. Due to the influence of the three-phase magnetic circuit system, the phase disconnection voltage is not 0, but it will decrease. The voltages of other phases are normal. [img=250,171]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t1-53.gif[/img] Figure 1 Single-power supply single-circuit disconnection operation: When one phase (C phase) is disconnected, 3 [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] a [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] b = -[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] c [/sub], that is, 3U0 = 33.3V; when two phases (B, C) are disconnected, 30 = a, that is, 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] a [/sub]. (4) Secondary one-phase (two-phase) disconnection: Due to the influence of the non-magnetic circuit system, the phase disconnection voltage is lower than that of the primary disconnection, while other phases are normal. 1.2 Voltage Calculation and Analysis During Line Disconnection (1) Single-phase disconnection of a single-source, single-circuit line In the system shown in Figure 1, the neutral points of the main transformers on the M and N sides are not grounded or grounded through the arc suppression coil. When phase A of line MN is disconnected, the boundary conditions are: A = 0; B [/sub]＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] C [/sub]＝0;Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [/sub]＝0；Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub]＝0 Expressing the above conditions using symmetric components: [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]＝[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A1＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/f iles/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A2+[img=13,21]http://zszl.ce pee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] 0 [/sub]＝0 [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] B [/sub]＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] C [/sub]＝α [sup][sup]2[/sup][/sup][img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img] A1＋α[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A2 +[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] 0 [/sub]+α[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A1+α [sup][sup]2[/sup][/sup][img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img] A2＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][size=2][sub]0[/sub] [/size] =-([img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A1＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/file s/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A2)+2[img=13,21]http://zszl.cep ee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] 0 [/sub]=0 Therefore[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A1＝-[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/fil es/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A2;[img=13,21]http://zszl.cep ee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] 0 [/sub]=0 And Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]A1＝（ Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]+αΔ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [/sub]+α[sup] 2 [/sup]Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub])／3＝Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]／3 Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]A2＝（ Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]＋α[sup] 2 [/sup]Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [/sub]+αΔ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub])／3＝Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]／3 Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]＝（Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]+Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [sub]＋Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub]）／3＝Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]／3 According to the above symmetrical component boundary conditions, the composite sequence network can be obtained as shown in Figure 2. In this single-power supply system, ΔA＝A－0＝A. After a single-phase disconnection occurs, the motor will stop rotating, and the comprehensive positive sequence impedance is equal to the negative sequence impedance. Since the neutral points of the main transformers on the M and N sides are not grounded or are grounded through the arc suppression coil, 0 = ∞. According to the composite sequence network: [img=302,40]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/g1-53.gif[/img] Therefore, Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]＝3Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img] A2＝3[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]／2. [img=249,196]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t2-53.gif[/img] Figure 2. In a single-power-source, single-circuit, single-phase disconnection composite sequence network, after phase A is disconnected, the capacitance of phase A to ground on the N side forms a capacitive current through the N-side main transformer, which will cause a voltage shift OM at the neutral point on the M side. Therefore, the N-side voltage is: [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]AN＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]AM－Δ[img=15,21]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]O M－3[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]／2＝－[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]／2＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]BN＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]BM－Δ[img=15,21]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] B [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM -0＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] B [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]CN＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]CM－Δ[img=15,21]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] C [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM -0＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] C [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]ON＝[img=15,21]http://zszl.cep ee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]AN＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/ wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]BN＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/ image/f-52.gif[/img]CN＝-3[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]／2+3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM So 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM－3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/file s/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]ON＝3[img=13,22]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]／2 As shown in the above formula, after phase A is disconnected, the magnitude of the voltage vector difference between the open deltas on both sides of M and N is 50 V. If the three-phase-to-ground capacitances are equal before the phase disconnection, then OM and ON are in opposite directions, and the sum of the open delta voltage values ​​on both sides of M and N is 50 V. The magnitude distribution of the open delta voltages on both sides depends on the location of the line disconnection. When there are many outgoing lines on side M and N is the terminal substation, the open delta voltage on side M is close to 0, and the open delta voltage on side N is close to 50 V. A grounding signal is issued on side N, and the voltage vector diagram on both sides is shown in Figure 3. [img=202,218]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t3-53.gif[/img] Figure 3. Voltage vector diagram on both sides when the A phase of a single-power supply single-circuit line is disconnected. After the disconnection, if the disconnected phase on the M side is successively grounded, the voltage of phase A on the M side drops to 0; the open delta voltage is 100 V, and a grounding signal is issued. The voltage of phase A on the N side rises to 3/2 times the phase voltage; the open delta voltage is 150 V, and a grounding signal is issued (see Figure 4 for the voltage vectors on both sides). [img=208,285]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t4-53.gif[/img] Figure 4. Voltage vector diagram on both sides when phase A of a single-power supply single-circuit line is disconnected and phase A of the M side is grounded. After the disconnection, if the disconnected phase on the N side is successively grounded, the voltage of phase A on the M side rises to 3/2 times the phase voltage, the open delta voltage is 50 V, and a grounding signal is generated. The voltage of phase A on the N side drops to 0, the open delta voltage is 0, and no grounding signal is generated (see Figure 5 for the voltage vectors on both sides). [img=206,195]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t5-55.gif[/img] Figure 5 Voltage vector diagram on both sides when phase A of a single-source single-circuit line is disconnected and phase A of the N side is grounded (2) Two-phase disconnection of a single-source single-circuit line In the system shown in Figure 1, if phase BC of line MN is disconnected, the boundary condition is: Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [sub]＝Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]A1＋Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]A2＋Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]＝0 [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] B [/sub]＝0;[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] C [/sub]＝0 And [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A1＝（ [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]＋α[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] B [/sub]＋α [sup]2[/sup][img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub]C[/sub] )／3＝[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]／3 [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A2＝（ [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]＋α [sup]2[/sup][img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub]B[/sub] +α[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] C [/sub])／3＝[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]／3 [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] 0 [/sub]＝（[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] B [sub]＋[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] C [/sub]）／3＝[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img][sub] A [/sub]／3 According to the above symmetric component boundary conditions, the composite sequence network can be obtained as shown in Figure 6. Its parameters, Δ[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]-0＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A The combined positive and negative sequence impedances are [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f3-53.gif[/img], [img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f3-53.gif[/img][sub] 0 [/sub]＝∞. [img=149,37]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/g1-54.gif[/img] Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub]  A2 = -[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A2[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f3-53.gif[/img] = 0 Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]A1＝[ img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [sub]－[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-53.gif[/img]A1[img=13,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f3-53.gif[/img]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][size=2][sub]A[/sub] [/size] Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]＝-Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub]  [/sub]A1＝-[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][size=2][sub]A[/sub] [/size] Therefore Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] B [/sub]－[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][size=2][sub]A[/sub] [/size] Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] C [sub]－[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub] [size=2][img=244,194]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t6-55.gif[/img] [/size] Figure 6 Composite sequence diagram of two-phase open circuit in a single power supply and single circuit. When the circuit is broken, the capacitance of B and C phases to ground on the N side forms a capacitive current through the N side main transformer, which will cause the neutral point on the M side to form a voltage shift OM, the direction of which is opposite to that of A. Then the voltage on the N side is: [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]AN＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]AM－Δ[img=15,21]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] A [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]BN＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]BM－Δ[img=15,21]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] B [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM [img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]CN＝[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]CM－Δ[img=15,21]http://zszl.cepe e.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] C [/sub]＝[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]ON＝[img=15,21]http://zszl.cep ee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]AN＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files /wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]BN＋[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]CN＝3[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][sub] A [/sub]＋3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM So 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904 /image/f-52.gif[/img]ON－3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hu beidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM＝3[img=13,22]http://zszl.cepee.com/cepee _kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img][size=2][sub]A[/sub] If the three-phase capacitance to ground is equal before the phase break, then after phases B and C are broken, the directions of OM and ON are opposite, meaning the sum of the open delta voltages on both sides of M and N is 100 V. The distribution of the open delta voltages on both sides depends on the location of the line break. When there are many outgoing lines on the M side and N is the terminal substation, the open delta voltage on the M side is close to 0, and the open delta voltage on the N side is close to 100 V, triggering a grounding signal on the N side. The voltage vector diagram on both sides is shown in Figure 7. Since the potentials of phases A, B, and C on the N side are equal to that of phase A on the M side, the voltage can be easily calculated from the vector diagram when single-phase grounding occurs successively in the broken phases. [img=188,252]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/t7-55.gif[/img] Figure 7 Voltage vector diagram on both sides when the two phases of a single-power-supply single-circuit line BC are disconnected (3) Disconnection of a single-power-supply double-circuit line Since the other circuit maintains normal electrical connection after the disconnection, in the composite sequence network, the potential difference Δ on both sides is equal to the voltage drop of the load current on the line when the single circuit is running, while Δ[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 If Δ[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img] is less than 15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img][sub] 0 [/sub]／2＜Δ[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img]／2。 Because the zero-sequence voltage on both sides of the line is less than the zero-sequence voltage between the two ends of the line, if the line voltage drop at the maximum load of a single circuit is 30% of the rated voltage, then the open delta voltage on both sides of the line is less than 15 V, while the setting operating voltage of the grounding relay is 15～30 V, so no grounding signal will be issued. (4) Dual power supply line disconnection For a dual circuit, the result is similar to that of a single power supply dual circuit, and no grounding signal will be issued. For a single circuit, the calculation method is similar to that of a single power supply single circuit. When a single phase is disconnected: 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM－[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/h ubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]ON≈3Δ[img=13,22]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img]／2 (approximately equal to the positive and negative sequence impedances are not equal) When two phases are disconnected: 3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]ON－3[img=15,21]http://zszl.cepee.com/cepee_kjlw_pic/ files/wx/hubeidl/hbdl99/hbdl9904/image/f-52.gif[/img]OM＝3Δ[img=13,22]http://zszl .cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9904/image/f2-53.gif[/img] After a single-phase disconnection, if the potential difference between the two power supplies is less than 30% of the rated voltage, or after a two-phase disconnection, if the potential difference between the two power supplies is less than 15% of the rated voltage, no grounding signal will be generated. [b]2 Fault Judgment[/b] In summary, when generating a grounding signal, different fault types and different fault characteristics result in different characteristics of the three-phase voltage and open delta voltage. 2.1 Single-phase grounding: Metallic single-phase grounding: One phase voltage is zero, and the voltages of the other two phases rise to the line voltage, with the open delta voltage equal to 100 V. Non-metallic grounding: One phase voltage is low and not zero; the voltages of the other two phases rise and approach the line voltage, with the open delta voltage less than 100 V. 2.2 Ferromagnetic resonant frequency divisional resonance: The three-phase voltages rise sequentially, and the voltmeter pointer shows low-frequency oscillation within the same range, generally not exceeding twice the phase voltage, with the open delta voltage generally less than 100 V. Fundamental resonance: One phase (two phases) voltage decreases and is not zero; the other two phases (one phase) voltage increases and is greater than the line voltage, generally not exceeding 3 times the phase voltage, and the open delta voltage is less than 100 V. High-frequency resonance: All three phase voltages increase simultaneously, and the increase is greater than the line voltage, generally not exceeding 3 to 3.5 times the phase voltage. The open delta voltage is greater than 100 V. 2.3 PT disconnection (1) One or two phases of the open delta winding are reversed: The three phase voltages are normal, and the open delta voltage is equal to 66.7 V. (2) The secondary neutral line is disconnected, and at the same time, a single phase of the primary system is grounded: The three phase voltages remain unchanged, and the open delta voltage is equal to 100 V. (3) One phase (two phases) of the primary circuit is disconnected: The voltage of one phase (two phases) decreases, the voltages of the other phases are normal, and the open delta voltage is equal to 33.3 V. 2.4 Single-power-source single-circuit line disconnection and successive grounding (1) Single-phase disconnection: The voltage of one phase on the power supply side rises, less than 3/2 times the phase voltage; the voltage of two phases drops, greater than 0.866 times the phase voltage; for the disconnection of the terminal line, the change is not significant. The voltage of one phase on the load side decreases, less than 0.5 times the phase voltage; the voltage of the other two phases decreases, greater than 0.866 times the phase voltage; for the disconnection of the terminal line, the change is not significant. The sum of the open delta voltages on both sides is 50 V (assuming that the three phases to ground capacitances are equal before the disconnection). When it is the disconnection of the terminal line, the voltage on the power supply side is close to 0, and the voltage on the load side is close to 50 V. (2) Single-phase disconnection and successive grounding on the power supply side: The voltage of one phase on the power supply side is 0, and the voltage of the other two phases rises to the line voltage, and the open delta voltage is equal to 100 V. The voltage of one phase on the load side rises to 3/2 times the phase voltage, and the voltage of the other two phases rises to the line voltage, and the open delta voltage is equal to 150 V. (3) Single-phase open circuit and successive grounding on the load side: The voltage of one phase on the power supply side rises to 3/2 times the phase voltage, and the voltage of the other two phases drops to 0.866 times the phase voltage. The open delta voltage is equal to 50 V. The voltage of one phase on the load side drops to 0, and the voltage of the other two phases drops to 0.866 times the phase voltage. The open delta voltage is equal to 0. (4) Two-phase open circuit: The voltage of one phase on the power supply side decreases, and the voltage of the other two phases increases. For the end line open circuit, the change is not significant. The voltage of all three phases on the load side decreases. For the end line open circuit, the change is not significant. The sum of the open delta voltages on both sides is 100 V. When the end line is open, the voltage on the power supply side is close to 0, and the voltage on the load side is close to 100 V. (5) Two-phase open circuit and successive single-phase grounding on the power supply side: The voltage of one phase on the power supply side is 0, and the voltage of the other two phases rises to the line voltage. The open delta voltage is equal to 100 V. The voltage of all three phases on the load side rises to the line voltage. The open delta voltage is equal to 173 V. (6) Two-phase disconnection and successive grounding on the load side: The voltage of one phase on the power supply side is 0, and the voltage of the other two phases rises to the line voltage, with the open delta voltage equal to 100 V. The voltage of all three phases on the load side drops to 0, with the open delta voltage equal to 0. In general, a PT disconnection usually only occurs on a section of busbar in a substation at a given time. When a single-phase grounding occurs, the entire low-current grounding system will experience the same voltage change; when a line is disconnected, the voltages on both sides are significantly different, and the line current also changes significantly; while for ferroresonance, the voltage change characteristics are particularly prominent. In addition, it is common for several faults to occur simultaneously, with PT disconnection being the most likely to occur simultaneously with other types of faults. Currently, most substations are not equipped with grounding protection line selection devices, requiring manual judgment and manual troubleshooting. Therefore, correctly judging the type and nature of the fault is crucial when issuing a grounding signal. First, a preliminary judgment should be made based on the three-phase phase voltage and open delta voltage of each busbar operating in parallel within the substation. Second, inquire about any abnormalities at other substations and further observe whether the instrument readings of the arc suppression coil, line voltage, and three-phase current are normal. If necessary, appropriate checks should be performed, such as checking whether the PT fuses and auxiliary contacts of the PT disconnectors are intact, and using a voltage tester to verify the voltage. As long as the above basic principles and characteristics are mastered, and experience in fault analysis is continuously accumulated, and applied flexibly in practical work, the fault type is not difficult to determine.