A Novel Busbar Protection Based on the Average Product of Fault Components

energies

Article A Novel Busbar Protection Based on the Average Product of Fault Components

Guibin Zou 1,* ID , Shenglan Song 2, Shuo Zhang 1 ID , Yuzhi Li 2 and Houlei Gao 1

1 Key Laboratory of Power System Intelligent Dispatch and Control of Ministry of Education, Shandong University, Jinan 250000, China; [email protected] (S.Z.); [email protected] (H.G.) 2 State Grid Weifang Power Supply Company, Weifang 261000, China; [email protected] (S.S.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel.: +86-135-0541-6354

Received: 13 March 2018; Accepted: 1 May 2018; Published: 3 May 2018

Abstract: This paper proposes an original busbar protection method, based on the characteristics of the fault components. The method firstly extracts the fault components of the current and voltage after the occurrence of a fault, secondly it uses a novel phase-mode transformation array to obtain the aerial mode components, and lastly, it obtains the sign of the average product of the aerial mode voltage and current. For a fault on the busbar, the average products that are detected on all of the lines that are linked to the faulted busbar are all positive within a specific duration of the post-fault. However, for a fault at any one of these lines, the average product that has been detected on the faulted line is negative, while those on the non-faulted lines are positive. On the basis of the characteristic difference that is mentioned above, the identification criterion of the fault direction is established. Through comparing the fault directions on all of the lines, the busbar protection can quickly discriminate between an internal fault and an external fault. By utilizing the PSCAD/EMTDC software (4.6.0.0, Manitoba HVDC Research Centre, Winnipeg, MB, Canada), a typical 500 kV busbar model, with one and a half circuit breakers configuration, was constructed. The simulation results show that the proposed busbar protection has a good adjustability, high reliability, and rapid operation speed.

Keywords: busbar protection; fault direction; fault components; average product; CT (current transformer) saturation

1. Introduction As one of the most significant elements in electric power systems, the busbar takes on the crucial task of the collection and distribution power. The failures or maloperation of the busbar protection device may bring about serious consequences. Furthermore, with the expansion of the scale of power systems and the continuous rise of voltage levels, it is very important to equip fast, sensitive, and reliable busbar protection. At present, the widely used busbar protection is the current differential protection [1–3], whose performance is affected by the current transformer (CT) saturation, CT ratio-mismatch, and so on. Moreover, the current differential protection requires trict sampling synchronization. In order to counteract the influence of the CT saturation, a series of algorithms for detecting the CT saturation are proposed in the works of [4–7], but these methods generally cause a delay in the operation time. Busbar protection techniques that are based on the transient current are described in the works of [8–11], and these techniques can achieve a fault detection before the CT saturation. However, they require a complex wavelet transform or mathematical morphology in order to extract the fault characteristics. In addition, the sensitivity and reliability of these protection methods are seriously influenced by a fault

Energies 2018, 11, 1139; doi:10.3390/en11051139 www.mdpi.com/journal/energies Energies 2018, 11, x FOR PEER REVIEW 2 of 16 Energies 2018, 11, 1139 2 of 16 influenced by a fault with the small inception angle. A novel traveling-wave-based amplitude integral busbar protection scheme is proposed by Zou et al. [12], and the simulation results with the small inception angle. A novel traveling-wave-based amplitude integral busbar protection demonstrate that it is rarely affected by the fault types, fault inception angles, CT saturation, etc., scheme is proposed by Zou et al. [12], and the simulation results demonstrate that it is rarely affected however, the high sampling frequency restricts the practicability of this method. In Song et al. [13], by the fault types, fault inception angles, CT saturation, etc., however, the high sampling frequency the authors proposed a new busbar protection method that is based on the polarity comparison of restricts the practicability of this method. In Song et al. [13], the authors proposed a new busbar the superimposed current, as a result of only using the fault component current, so the noise protection method that is based on the polarity comparison of the superimposed current, as a result disturbance has a negative influence on the reliability of this method. of only using the fault component current, so the noise disturbance has a negative influence on the A directional transmission line protection technique and a rapid busbar protection, using the reliability of this method. average of superimposed components, are put forward in the work of Hashemi et al. and Song et al. A directional transmission line protection technique and a rapid busbar protection, using [14,15], respectively. However, the authors only analyze and simulate a simple single busbar the average of superimposed components, are put forward in the work of Hashemi et al. and configuration in Song et al. [15]. Referring to their ideas, the paper presents an original busbar Song et al. [14,15], respectively. However, the authors only analyze and simulate a simple single protection technique, according to the polarity differences of the average products on all of the busbar configuration in Song et al. [15]. Referring to their ideas, the paper presents an original busbar branches that are connected to the busbar. The main principle can be depicted as described below. protection technique, according to the polarity differences of the average products on all of the branches For a fault inside the busbar, during a specific duration of the post-fault, the detected average that are connected to the busbar. The main principle can be depicted as described below. For a fault products on all of the lines that are linked to the faulted busbar are positive. For a fault occurring on inside the busbar, during a specific duration of the post-fault, the detected average products on all of any one of these lines, during a specific time of the post-fault, the average product on the faulted the lines that are linked to the faulted busbar are positive. For a fault occurring on any one of these lines, line is negative, while those on the other non-faulted lines are positive. Taking advantage of these during a specific time of the post-fault, the average product on the faulted line is negative, while those characteristic differences, a novel busbar protection criterion is established. In order to testify to the on the other non-faulted lines are positive. Taking advantage of these characteristic differences, a novel effectiveness and practicability of the presented busbar protection, an effective 500 kV substation busbar protection criterion is established. In order to testify to the effectiveness and practicability of busbar model with one and a half circuit breakers configuration was built and extensive the presented busbar protection, an effective 500 kV substation busbar model with one and a half simulations were implemented. Moreover, some of the influencing factors have also been circuit breakers configuration was built and extensive simulations were implemented. Moreover, some discussed. of the influencing factors have also been discussed.

2. Principle of Busbar Protection

2.1. Fault Analysis

A fault circuit of simple busbar structure is illustrated in Figure 1, where there are lines l1 to l3, A fault circuit of simple busbar structure is illustrated in Figure1, where there are lines l1 to which are connected to busbar B. R1, R2, and R3 are the detection units at the start of each line. The l3, which are connected to busbar B. R1,R2, and R3 are the detection units at the start of each line. Theforward forward direction direction of the of current the current for each for each line lineis defined is deﬁned as flowing as ﬂowing from from the thebusbar busbar to the to theline. line.

Figure 1. Fault circuit of a simple busbar structure.

Supposing that each line is lossless, which does not produce a negative influence on fault Supposing that each line is lossless, which does not produce a negative inﬂuence on fault analysis, analysis, and L1, L2, and L3 represent the equivalent inductances of l1, l2, and l3, respectively. On the and L , L , and L represent the equivalent inductances of l , l , and l , respectively. On the basis of the basis 1of 2the superposition3 principle, the fault superimposed1 2 circuit3 can be modeled by a fault superposition principle, the fault superimposed circuit can be modeled by a fault superimposed voltage superimposed voltage source, which has same amplitude and opposite sign, compared with the source, which has same amplitude and opposite sign, compared with the pre-fault voltage of the pre-fault voltage of the fault point. The pre-fault voltage is set as VF = Umsin(ωt + θ), and θ denotes fault point. The pre-fault voltage is set as V = Umsin(ωt + θ), and θ denotes the fault inception angle. the fault inception angle. The fault analysisF is based on a single-phase system that is conducted, as The fault analysis is based on a single-phase system that is conducted, as shown below, for simplicity shown below, for simplicity and convenience. and convenience.

2.1.1. Internal Busbar Fault

For a single-phase to ground fault, which occurrs at f1 on busbar B, the corresponding fault For a single-phase to ground fault, which occurrs at f 1 on busbar B, the corresponding fault superimposed network is demonstrateddemonstrated inin FigureFigure2 2a.a.

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Δ i1 Δ i1 Δ u1 Δ u1

(a) (b)

Figure 2. The (a) fault superimposed network when a fault at f1 on busbar B occurs. The (b) fault Figure 2. The (a) fault superimposed network when a fault at f 1 on busbar B occurs. The (b) fault superimposed network when a fault at f2 on l1 occurs. superimposed network when a fault at f 2 on l1 occurs.

From Figure 2a, the fault component voltage Δu1, fault component current Δi1, and their From Figure2a, the fault component voltage ∆u1, fault component current ∆i1, and their corresponding average product, that is detected by R1, are expressed as follows. corresponding average product, that is detected by R1, are expressed as follows. Δ=−=−ωθ + ut1 () VFm U sin( t ) (1) ∆u1(t) = −VF = −Um sin(ωt + θ) (1) Δ dit1 () LVUtd∆i1(t) =− =−sin(ωθ + ) (2) L 1 = −V Fm= −U sin(ωt + θ) (2) 1 dt dt F m

FromFrom (2), (2), Δ∆ii11((tt)) can can be be obtained obtained as as follows: follows: UU ∆iΔ=−(t) = − mm(cos θθωθ− −cos(ωt + +θ)) (3) 1 it1 ( ) (cos cos( t )) (3) X1 X1

So, the average values of ∆u1(t) and ∆i1(t) are as below: So, the average values of Δu1(t) and Δi1(t) are as below:

1 R t ave(∆u (t)) = 1 t ∆u (τ)dτ ave(())Δ=Δ1 u tT 0 u1 ()ττ d (4) 11= − Um ( − ( + )) T 20π cos θ cos ωt θ (4) t U ave(∆i ( t )) = 1 R =−∆i (τ)md (cosτ θωθ − cos(t + )) 1 T 0 1 π (5) = − Um (ω2t cos θ − sin(ωt + θ) + sin θ) 2πX1

Finally, the average product is deduced1 t from Equations (4) and (5), as follows: ave(())Δ=Δ i t i ()ττ d 11T 0 S1 = ave(∆u1(t)) × ave(∆i1(t)) (5) 2 UmU m ==−2 (cos (ωθθtt cos− cos −(ω sin(t + ωθθ)) + ) + sin θ ) (6) 4π πX1 ×(ω2t cosX1θ − sin(ωt + θ) + sin θ) Finally, the average product is deduced from Equations (4) and (5), as follows: where X1 = ωL1, ave(∆u1) and ave(∆i1) denote the average values of the fault component voltage and fault component current, respectively.=ΔS is equal to ×Δ the product of ave(∆u ) and ave(∆i ). ω is the angle Saveutaveit111 (())(()) 1 1 1 frequency, and T represents the power frequency2 cycle. Using Equations (1), (3) and (6), the waveforms Um of ∆u1, ∆i1, and S1, under several typical=−+ fault inception(cosθωθ cos( angles,t are )) depicted in Figure3. All of the(6) 4π 2 X electric parameters, in Figure3, are expressed by1 the per-unit form. ×−++(cossin(ωθtt ωθ )sin) θ where X1 = ωL1, ave(Δu1) and ave(Δi1) denote the average values of the fault component voltage and fault component current, respectively. S1 is equal to the product of ave(Δu1) and ave(Δi1). ω is the angle frequency, and T represents the power frequency cycle. Using Equations (1), (3) and (6), the waveforms of Δu1, Δi1, and S1, under several typical fault inception angles, are depicted in Figure 3. All of the electric parameters, in Figure 3, are expressed by the per-unit form.

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FigureFigure 3. 3.∆ uΔ1u,1∆, Δi1i,1, and andS S1 1detected detectedby byR R11 forfor anan internalinternal busbarbusbar fault.fault.

As shown in Figure 3, S1 is positive during the specific interval of the post-fault, which means a As shown in Figure3, S1 is positive during the specific interval of the post-fault, which means a backward fault for l1. In other words, if 0 ≤ θ < π, S1 is positive at 0 ≤ t < T(π − θ)/π and if π ≤ θ < 2π, backward fault for l1. In other words, if 0 ≤ θ < π, S1 is positive at 0 ≤ t < T(π − θ)/π and if π ≤ θ < 2π, S1 is positive at 0 < t < T(2π − θ)/π. Here, T is equal to 20 ms. S1 is positive at 0 < t < T(2π − θ)/π. Here, T is equal to 20 ms. Similarly, analyzing the average products that are detected by R2 and R3, show identical Similarly, analyzing the average products that are detected by R2 and R3, show identical characteristics to those that are detected by R1. In a word, for a fault occurring on the busbar, the characteristics to those that are detected by R1. In a word, for a fault occurring on the busbar, the average productaverage that product is detected that is ondetected each line on iseach positive line is within positive a speciﬁc within timea specific of the time post-fault. of the post-fault.

2.1.2.2.1.2. External External Fault Fault

ForFor a a single-phase single-phase fault fault at fat2 on f2 lon1, thel1, the fault fault superimposed superimposed network network is shown is shown in Figure in 2Figureb, in which 2b, in Lwhich11 represents L11 represents the equivalent the equivalent inductance inductance from busbar from Bbusbar to the faultB to the location, fault location, and L1 minus and LL1 11minusequals L11 Lequals12. For Ll112,. the For fault l1, the components fault components and average and average product product detected detected by R1 can by beR1 writtencan be written as below. as below. XU Δ=−X2//32//3Um m ωθ + ∆u (t)ut1 =()− sin sin((ωt t+ θ) ) (7)(7) 1 X0X '

Um ∆i1(t) = U(cos θ − cos(ωt + θ)) (8) Δ=it( )X0 m (cosθωθ − cos( t + )) (8) 1 X ' Using Equations (7) and (8), the average product can be obtained, as follows: Using Equations (7) and (8), the average product can be obtained, as follows: S1 = ave(∆u1(t)) × ave(∆i1(t)) Saveutaveit=Δ( (2 )) ×Δ ( ( )) 11= − X2//3Um ( − 1( t + )) (9) 4π2X02 cos θ cos ω θ XU2 ×(ω=−t cos2//3θ +msin(cosθ −θωθsin − cos((ωt +t +θ)) )) (9) 4π 2'2X where, X2//3 = ωL2//3, L2//3 = L2L3/(L×+−+2(cossinsin(ωθ+ttL3), X’ = θωL’, and ωθL’ ))= L11 + L2//3. According to the equations that are mentioned above, the waveforms of ∆u1, ∆i1, and S1 are drawn in Figure4. 2//3 2//3 2//3 2 3 2 3 11 2//3 where,From X Figure= ωL4, the, L following= L L /(L conclusions+ L ), X’ = ωL can’, and be obtained,L’ = L + L namely:. According if 0 ≤ toθ < theπ, equationsS1 is negative that for are mentioned above, the waveforms of Δu1, Δi1, and S1 are drawn in Figure 4. 0 ≤ t < T(π − θ)/π and if π ≤ θ < 2π, S1 is negative for 0 < t < T(2π − θ)/π. From Figure 4, the following conclusions can be obtained, namely: if 0 ≤ θ < π, S1 is negative for Similarly, for a fault that is occurring on l2 or l3, the average product that is detected by R2 or R3 0 ≤ t < T(π − θ)/π and if π ≤ θ < 2π, S1 is negative for 0 < t < T(2π − θ)/π. has the identical characteristics to those that are detected by R1. In summary, for a fault occurring on the line,Similarly, during thefor speciﬁca fault that period is occurring of the post-fault, on l2 or thel3, the detected average average product product that is of detected the faulted by R line2 or isR3 has the identical characteristics to those that are detected by R1. In summary, for a fault occurring on the line, during the specific period of the post-fault, the detected average product of the faulted line

Energies 2018, 11, 1139 5 of 16 Energies 2018, 11, x FOR PEER REVIEW 5 of 16 negative,is negative, which which denotes denotes a forwarda forward fault, fault, but but the the average average products products on on the the non-faulted non-faulted lineslines thatthat areare linkedlinked toto thethe samesame busbarbusbar areare positive,positive, whichwhich meansmeans backward backward faults. faults.

FigureFigure 4. 4.∆ Δuu1,1,∆ Δi1i1,, andand SS11 detecteddetected byby RR11 for a fault on l1..

2.2.2.2. PhasePhase ModeMode TransformationTransformation TheThe analyses that that are are mentioned mentioned above above are are based based on ona single-phase a single-phase power power system. system. In an Inactual an actualthree-phase three-phase system, system, the electromagnetic the electromagnetic coupling coupling between between phase phaseand phase and phasecan be caneliminated be eliminated by the byphase-mode the phase-mode transformation transformation technique, technique, such suchas asthe the Clarke Clarke transformation transformation and and KarenbauerKarenbauer transformation.transformation. However, thesethese transformationtransformation techniquestechniques have an inherentinherent disadvantage,disadvantage, thatthat is,is, thethe singlesingle modemode componentcomponent failsfails toto expressexpress allall ofof thethe faultfault types. types. Therefore,Therefore, aa newnew phase-modephase-mode transformationtransformation arrayarray TT isis appliedapplied herehere inin SongSong [[13].13].  111 1 1 1 =− − T = T −1 145−4 5  (10)(10)   −1−−15 5 −4 4

ByBy applyingapplying TT intointo thethe three-phasethree-phase system,system, thethe modemode componentscomponents cancan bebe obtainedobtained fromfrom thethe phasephase components,components,      y0 1 1 1 ya yy111  y =0  −1 −4 5 a y  (11)  1    b  =−− − − y2 yy1 1145 5 4 yc (11) b  where y , y , and y are either they fault component−−15 4 currents or fault component voltages. a b c 2 yc From Equation (11), it can be seen that y1 or y2 is capable of reﬂecting all of the fault types. where ya, yb, and yc are either the fault component currents or fault component voltages. From 2.3.Equation Busbar (11), Protection it can be Identiﬁcation seen that y Criterion1 or y2 is capable of reflecting all of the fault types. In the light of the fault analyses that are mentioned above, during a certain period of the post-fault, 2.3. Busbar Protection Identification Criterion the conclusions can be drawn as follows. In the light of the fault analyses that are mentioned above, during a certain period of the post-fault, the conclusions can be drawn as follows. 1. For a fault occurring on the line, the average product of the fault component voltage and fault component current, at the beginning of this line, is negative; but if a backward direction fault to

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1. For a fault occurring on the line, the average product of the fault component voltage and fault component current, at the beginning of this line, is negative; but if a backward direction fault to this line occurs, the average product of the fault component voltage and fault component current is positive. This conclusion is suitable for any one of the lines that are linked to the identical busbar. 2. In case of a fault on the busbar, the average products of the fault component voltage and fault componentEnergies 2018 current, 11, x FOR atPEER the REVIEW beginning of all of the lines that are linked to the faulted6 of 16 busbar are positive. this line occurs, the average product of the fault component voltage and fault component Accordingcurrent to the is aforementionedpositive. This conclusion conclusions, is suitable a for criterion any one that of the identifies lines that the are faultlinked direction to the can be identical busbar. constructed2. asIn follows: case of a fault on the busbar, the average products of the fault component voltage and fault component current at the beginningSm = ave of (all∆ uofm the) × linesave (that∆im are) linked to the faulted busbar are (12) positive. where ave(∆um) and ave(∆im) denote the average values of the aerial mode voltage and aerial mode According to the aforementioned conclusions, a criterion that identifies the fault direction can current thatbe areconstructed detected as follows: at the beginning of the mth line, respectively. In practical application, they can be discrete, as follows: Saveuavei=Δ×Δ() () (12) m 1mmj ave(∆u ) = ∆u (k) (13) where ave(Δum) and ave(Δim) denote the averagem values∑ of them aerial mode voltage and aerial mode N = current that are detected at the beginning of the mthk line,1 respectively. In practical application, they can be discrete, as follows: 1 j ave(∆im) = j ∆im(k) (14) Δ=N1∑ Δ ave() ummk=1 u ()k (13) N k=1 where, j is the sample numbers that are used to compute, and N is the sample number in a per cycle. j If Sm is negative, a forward fault would be identifiedΔ=1 for Δ the mth line. Otherwise, if Sm is positive, ave() imm i ()k (14) the fault direction is backward. In the case of a busbarN k linked=1 to the n lines, the corresponding busbar protectionwhere, criteria j is canthe sample be expressed numbers that as follows: are used to compute, and N is the sample number in a per cycle. If Sm is negative, a forward fault would be identified for the mth line. Otherwise, if Sm is positive, the n fault direction is backward. In the case of a busbar linked to the n lines, the corresponding busbar λ = sign(S ) (15) protection criteria can be expressed as follows:∑ m m=1 n λ = sign() S where sign(S ) denotes the sign of S , and if S > 0, sign(mS ) = 1; if S = 0, sign(S ) = 0; and(15) if S < 0, m m m m=1 m m m m sign(Sm) = −1. If λ is equal to n, a fault inside the busbar would be identified, or else an external fault where sign(Sm) denotes the sign of Sm, and if Sm > 0, sign(Sm) = 1; if Sm = 0, sign(Sm) = 0; and if Sm < 0, would be discriminated.sign(Sm) = −1. If λ is equal to n, a fault inside the busbar would be identified, or else an external fault would be discriminated. 3. Identification Procedure of Busbar Protection 3. Identification Procedure of Busbar Protection On the basis of the principle that is proposed in Section2, for a busbar with n lines, On the basis of the principle that is proposed in Section 2, for a busbar with n lines, the the identification flowchart of the proposed protection is illustrated in Figure5. identification flowchart of the proposed protection is illustrated in Figure 5.

Figure 5. Identification flowchart of the busbar fault. Figure 5. Identiﬁcation ﬂowchart of the busbar fault.

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As shown in Figure5, the procedure for identifying busbar fault is as follows:

1. The protection adopts the startup elements, based on the change of the voltage and the current, and the protection starts when three successive samples satisfy Equations (16) or (17): Energies 2018, 11, x FOR PEER REVIEW 7 of 16

∆ip(k) = ip(k) − ip(k − N) ≥ 0.2IN (16) As shown in Figure 5, the procedure for identifying busbar fault is as follows: 1. The protection adopts the startup elements, based on the change of the voltage and the current, ∆up(k) = up(k) − up(k − N) ≥ 0.1UN (17) and the protection starts when three successive samples satisfy Equations (16) or (17): where, p represents phase A, phase B, or phase C, ip(k) and up(k) denote the kth sample of the Δ−−≥ik()= ik () ik ( N ) 0.2 I (16) fault current and voltage, respectively;p pp∆ip(k) and ∆up(k N) denote the kth sample of the fault component current and voltage, respectively; and IN and UN are the rated phase current and Δ−−≥= voltage, respectively. When oneuk ofp () all ofupp the () k startup uk ( N elements ) 0.1 U N are connected to the bus(17) operates, the relevant startup elements operate. where, p represents phase A, phase B, or phase C, ip(k) and up(k) denote the kth sample of the 2. Once a fault is detected, the busbar protection extracts the fault component voltage and fault fault current and voltage, respectively; Δip(k) and Δup(k) denote the kth sample of the fault componentcomponent current current for eachand voltage, line that respectively; is connected and toIN theand protectedUN are the busbarrated phase at once. current and 3. Then,voltage, the fault respectively. component When voltage one of all andof the fault startup component elements are currentconnected of to the bus three operates, phases are the relevant startup elements operate. transformed to the mode components, according to Equation (11), and the aerial mode y1 will be used2. toOnce calculate a fault theis detected, average the product. busbar protection extracts the fault component voltage and fault component current for each line that is connected to the protected busbar at once. 4. Further,3. Then, using the Equations fault component (12)–(15), voltage the aveand( ∆faultum), componentave(∆im), S currentm, and λof canthe bethree obtained. phases are 5. Finally,transformed according to to the the mode value components, of λ, a fault according inside orto Equation outside (11), the busbarand the aerial will be mode determined y1 will be by the protectionused method.to calculate the average product. 4. Further, using Equations (12)–(15), the ave(Δum), ave(Δim), Sm, and λ can be obtained. 4. Simulation5. Finally, and according Analyses to the value of λ, a fault inside or outside the busbar will be determined by the protection method. 4.1. Simulation Model 4. Simulation and Analyses According to an effective 500 kV substation configuration and parameters, a busbar simulation model was4.1. Simulation built to testifyModel to the effectiveness and practicability of the proposed busbar protection technique, asAccording shown in to Figurean effective6. Busbar 500 kV I substation and busbar configuration II were linked and parameters, by three series a busbar circuit simulation breakers and the modelmodel included was built four to testify lines to and the two effectiveness transformer and practicability branches. Theof the researched proposed busbar objects protection were busbar I technique, as shown in Figure 6. Busbar I and busbar II were linked by three series circuit breakers and busbar II. R1,R3, and R5 formed a group to protect the busbar I. They utilized the currents of CT1, and the model included four lines and two transformer branches. The researched objects were CT3, and CT5, and the voltages of PT1 (potential transformer), PT3, and PT5, respectively. While R2, busbar I and busbar II. R1, R3, and R5 formed a group to protect the busbar I. They utilized the R4, andcurrents R6 extracted of CT the1, CT currents3, and CT of5, CT and2, CTthe4 ,voltages and CT 6ofand PT1 the(potential voltages transformer), of PT2, PT 4PT, and3, and PT PT6, and5, then discriminatedrespectively. therunning While R2, stateR4, and of R busbar6 extracted II. the The currents power of lines CT2, CT adopted4, and CT the6 and frequency-dependent the voltages of PT2, and transposition-uniformPT4, and PT6, and parameters.then discriminated The the length running of eachstate of line busbar is indicated II. The power in Figure lines adopted6. Defining the the positivefrequency-dependent direction currentfor and each transposition-uniform line was the flow parameters. from the The busbar length to of line. each The line samplingis indicated frequency in that wasFigure used 6. in Defining the simulation the positive was direction set at current 4 kHz. fo Tenr each successive line was the samples flow from of the the busbar aerial to components line. The sampling frequency that was used in the simulation was set at 4 kHz. Ten successive samples of were usedthe foraerial the components fault detection. were used The for instant the fault of the detection. fault occurrence The instant wasof the generally fault occurrence at 0.5 mswas without special explanation.generally at 0.5 ms without special explanation.

Figure 6. Simulation model. Figure 6. Simulation model.

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4.2. Adaptability Analysis of the Proposed Protection Principle Energies 2018, 11, x FOR PEER REVIEW 8 of 16 In Section2, a busbar protection principle was proposed, and the theoretical analysis showed that the busbar4.2. Adaptability protection Analysis criteria of couldthe Proposed discriminate Protection between Principle the internal faults and external faults for a single busbarIn Section conﬁguration. 2, a busbar However, protection whether principle the was proposed proposed, protection and the theoretical principle wasanalys suitableis showed for the complexthat busbarthe busbar conﬁgurations protection criteri or not,a could for discriminate example, that between shown the in internal Figure 6fault, neededs and external further analysis.faults Notefor that a thesingle busbar busbar I andconfiguration busbar II. wereHowever, directly whether connected the proposed and very protection close because principle all was of suitable the middle circuitfor breakers the complex were busbar generally configuration closed ins theor not, normal for example, operation that status. shown in Figure 6, needed further analysis. Note that the busbar I and busbar II were directly connected and very close because all of 4.2.1.the Internal middle Fault circuit breakers were generally closed in the normal operation status.

If4.2 a.1 fault. Internal was Fault to occur at f 1 on busbar I, the similar analysis as that of the single busbar configuration was as follows. Within the certain duration of the post-fault, the average products If a fault was to occur at f1 on busbar I, the similar analysis as that of the single busbar that wereconfiguration detected was by as R 1follows.,R3, and Within R5 were the certain all positive, duration which of the post indicated-fault, t ahe fault average inside products the busbar that I. R2,R4were, and detected R6 should by R have1, R3, alland identified R5 were all as positive, the forward which faults indicated in theory, a fault but inside it was the unlikelybusbar I. thatR2, R all4, of themand could R6 correctlyshould have identify all identified the fault as directions,the forward becausefaults in thetheory busbar, but it I andwas IIunlikely were directly that all of connected. them Evencould so, the correctly final identification identify the result fault direction was stills, correct, because because the busbar only I ifand the II discrimination were directly connected results of. the R2,R4Even, and so, R 6thewere final all identification backward faults, result couldwas still the correct, busbar because II be considered only if the asdiscrimination an internal results fault. of the R2, R4, and R6 were all backward faults, could the busbar II be considered as an internal fault. 4.2.2. External Fault 4.2.2. External Fault For a fault at f 2 on l2, the theoretical analysis demonstrated that the average products that were For a fault at f2 on l2, the theoretical analysis demonstrated that the average products that were detected by R1 and R2 were all negative during the specific time interval of the post-fault, so both the detected by R1 and R2 were all negative during the specific time interval of the post-fault, so both the busbar I and busbar II were thought of as normal. busbar I and busbar II were thought of as normal. To summarize, the proposed busbar protection criterion should be fit for one and a half breakers To summarize, the proposed busbar protection criterion should be fit for one and a half of thebreakers busbar ofconfiguration. the busbar configuration. To verify the To correctness verify the correctnessof above analysis, of above the analysis, following the following simulations weresimulations executed. were executed.

4.3. Simulation4.3. Simulation of Typical of Typical Fault Fault

4.3.1.4 Internal.3.1. Internal Fault Fault ◦ As shownAs shown in Figure in Figure6, for 6, for a phase a phase A A grounding grounding fault, fault, at f1f 1withwith a fault a fault inception inception angle angle 30° and 30 aand a faultfault resistance resistance 25 25Ω ,Ω, Figures Figures7 and7 and8 show8 show the the waveforms waveforms ofof thethe aerialaerial mode mode voltage voltage and and aerial aerial modemode currents currents that were that detected were detected on each on line each of the line busbars of the I busbarand II,srespectively. I and II, respectively. The corresponding The averagecorresponding products are average shown products in Table are1. shown in Table 1.

Figure 7. The aerial mode components that were detected by R1, R3, and R5 for a fault at f1 on busbar I: Figure 7. The aerial mode components that were detected by R1,R3, and R5 for a fault at f 1 on busbar I: thethe (a) aerial(a) aerial mode mode voltage voltage and and the the ((bb)) aerial mode mode current current..

Energies 2018, 11, 1139 9 of 16 Energies 2018, 11, x FOR PEER REVIEW 9 of 16

Figure 8. The aerial mode components that were detected by R2, R4, and R6 for a fault at f1 on busbar I: Figure 8. The aerial mode components that were detected by R2,R4, and R6 for a fault at f 1 on busbar the (a) aerial mode voltage and the (b) aerial mode current. I: the (a) aerial mode voltage and the (b) aerial mode current. Table 1. Simulation results for a typical internal fault on busbar I. Table 1. Simulation results for a typical internal fault on busbar I. Busbar Protection Unit S(kVA) Fault Direction λ Identification Result Busbar ProtectionR1 Unit S(kVA)1019.8 FaultBackward Direction λ Identification Result I R3 892.7 Backward 3 Internal Fault R1 1019.8 Backward R5 650.6 Backward I R3 892.7 Backward 3 Internal Fault R2 192.7 Backward R5 650.6 Backward II R4 −106.5 Forward −1 External Fault R2 192.7 Backward R6 −86.8 Forward II R4 −106.5 Forward −1 External Fault As shown in FiguresR6 7 and 8, within−86.8 the specific Forward time interval of the post-fault, the aerial mode voltages and currents for R1, R3, and R5 were all positive. At the same time, the aerial mode voltages Asfor shownR2, R4, and in Figures R6 were7 also and positive,8, within however the specific the aerial time mode interval currents of the that post-fault, were detected the aerial by R2, mode R4, and R6 were not all positive. From Table 1, it can be seen that R1, R3, and R5 had all identified the fault voltages and currents for R1,R3, and R5 were all positive. At the same time, the aerial mode voltages direction as backward, so a fault inside the busbar I could be determined. However, the detection for R ,R , and R were also positive, however the aerial mode currents that were detected by R ,R , 2units4 on busbar6 II verified that busbar II was normal. Therefore, the discrimination results were2 4 and R were not all positive. From Table1, it can be seen that R ,R , and R had all identified the fault accurate6 . 1 3 5 direction as backward, so a fault inside the busbar I could be determined. However, the detection units on busbar4.3.2. IIExternal verified Fault that busbar II was normal. Therefore, the discrimination results were accurate.

4.3.2. ExternalAssuming Fault that a fault of the phase A grounding fault occurred at f2 from busbar II at 20 km, the corresponding waveforms of the aerial mode components are depicted in Figures 9 and 10, and the Assumingsimulation data that are a fault listed of in the Table phase 2. A grounding fault occurred at f 2 from busbar II at 20 km, the corresponding waveforms of the aerial mode components are depicted in Figures9 and 10, and the simulation data are listed in TableTable2 2.. Simulation results for a typical external fault. Busbar Protection Unit S (kVA) Fault Direction λ Identification Result Table 2. Simulation results for a typical external fault. R1 −360.7 Forward I R3 295.5 Backward 1 External Fault Busbar Protection Unit S (kVA) Fault Direction λ Identiﬁcation Result R5 62.8 Backward − R1R2 360.7−845.1 ForwardForward I R3 295.5 Backward 1 External Fault II R4 422.3 Backward 1 External Fault R5 62.8 Backward R6 419.9 Backward R2 −845.1 Forward II R4 422.3 Backward 1 External Fault

R6 419.9 Backward Energies 2018, 11, 1139 10 of 16 Energies 2018, 11, x FOR PEER REVIEW 10 of 16 Energies 2018, 11, x FOR PEER REVIEW 10 of 16

Figure 9. The aerial mode components that were detected by R1, R3, and R5 for an external fault: the Figure 9. The aerial mode components that were detected by R ,R , and R for an external fault: the (Figurea) aerial 9. modeThe aerial voltage mode and components the (b) aerial that mode were current detected. by R11, R3, and R5 5for an external fault: the (a) aerial(a) aerial mode mode voltage voltage and and the the (b) ( aerialb) aerial mode mode current. current.

Figure 10. The aerial mode components that were detected by R2, R4, and R6 for an external fault: the Figure 10. The aerial mode components that were detected by R2, R4, and R6 for an external fault: the Figure(a 10.) aerialThe mode aerial voltage mode and components the (b) aerial that mode were current detected. by R2,R4, and R6 for an external fault: the (a) aerial mode voltage and the (b) aerial mode current. (a) aerial mode voltage and the (b) aerial mode current. From Figures 9 and 10 and Table 2, it could be seen, within the short time of post-fault, that the From Figures 9 and 10 and Table 2, it could be seen, within the short time of post-fault, that the sign of the aerial mode current for R1 and R2 was opposite to that of the aerial mode voltage, so R1 sign of the aerial mode current for R1 and R2 was opposite to that of the aerial mode voltage, so R1 Fromand R Figures2 had identified9 and 10 the and fault Table direction2, it could as forward. be seen, Even within if the the other short protection time of post-fault,units had verified that the sign ofthatand the R the2 aerial had fault identified mode directions current the werefault for direction all R 1 backward,and Ras2 forward.was according opposite Even to if to thethe that criterionother of theprotection of aerial the busbarmodeunits had voltage, protection, verified so R1 that the fault directions were all backward, according to the criterion of the busbar protection, and Rfinally,2 had identifiedan external the fault fault was direction discriminated as forward. for both Evenbusbar ifs theI and other II. Therefore, protection the units discrimination had verified that theresultsfinally, fault werean directions external all right fault were. was all backward,discriminated according for both tobusbar the criterions I and II. of Therefore, the busbar the protection, discrimination finally, results were all right. an external fault was discriminated for both busbars I and II. Therefore, the discrimination results were 4.4. Simulation for Fault with Diverse Initial Conditions all right.4.4. Simulation for Fault with Diverse Initial Conditions Generally, the performances of protection are affected by the fault initial conditions, such as the 4.4. Simulationfault Generallyinception for Fault ,angle, the withperformances fault Diverse resistance, Initial of protection and Conditions fault aretype affected [16]. The by following the fault initialsimulations conditions, were suchcarried as outthe infault order inception to evaluate angle, the fault impact resistance, of the faultand fault initial type conditions [16]. The on following the method simulations. were carried out Generally,in order to theevaluate performances the impactof of protectionthe fault initial are conditions affected by on the the fault method initial. conditions, such as the fault inception angle, fault resistance, and fault type [16]. The following simulations were carried out in order to evaluate the impact of the fault initial conditions on the method. Energies 2018, 11, 1139 11 of 16

4.4.1. Different Fault Inception Angles

In general, for single-phase-grounding faults at f 1 on busbar I or at f 2 on l2 with different fault inception angles, Tables3 and4 show the corresponding simulation data respectively.

Table 3. Simulation results of the different fault inception angles for the busbar I faults.

Ag 0◦ Ag 15◦ Bg 0◦ Bg 200◦ Cg 0◦ Cg 310◦ Unit S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result

R1 369.2 813.9 3632.2 19,666.7 7369.9 45,005.9 R3 316.8Internal 704.0Internal 3076.9Internal 17,070.0Internal 6286.4Internal 39,859.1 Internal R5 237.5 523.8 2220.8 12,244.6 4588.0 28,234.1

R2 66.8 148.5 721.8 3877.5 1422.7 8945.1 R4 −45.5 External −95.5 External −427.4 External −2013.0 External −868.7 External −3974.4 External R6 −22.1 −54.5 −306.3 −1886.1 −574.4 −4946.9

Table 4. Simulation results of the different fault inception angles for the external faults.

Ag 0◦ Ag 30◦ Bg 0◦ Bg 60◦ Cg 0◦ Cg 200◦ Unit S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result

R1 −83.3 −164.0 −833.2 −5163.7 −1653.4 −2456.1 R3 63.5External 128.8External 691.4External 4268.7External 1331.7External 1995.9 External R5 19.4 34.1 135.4 887.3 313.6 443.1

R2 −195.0 −386.6 −1994.1 −12,152.6 −3918.3 −5833.9 R4 91.1External 184.3External 987.7External 6117.4External 1902.3External 2861.7 External R6 103.4 200.9 998.7 6019.6 2005.2 2951.3

The simulation data that are listed in Tables3 and4 veriﬁed that the proposed technique could effectively discriminate between the internal faults and external faults with diverse fault inception angles. Moreover, the method could identify the faults with a zero inception angle in a high sensitivity.

4.4.2. Different Fault Resistances

Setting the phase A and phase B grounding faults at f 1 on busbar I with fault resistances of 50 Ω, 150 Ω, and 300 Ω, and the phase A and phase C grounding faults at f 2 on l2 with fault resistances 0 Ω, 200 Ω, and 300 Ω, respectively, the simulation data are indicated in Table5.

Table 5. Simulation results of the different fault resistances for the busbar I faults and external faults.

ABg at f 1 ACg at f 2 Unit 50 Ω 150 Ω 300 Ω 0 Ω 200 Ω 300 Ω S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result

R1 16,557.0 4037.7 1298.1 −3297.8 −314.0 −171.3 R3 14,646.4Internal 3596.3Internal 1159.9Internal 2673.6External 251.9External 136.4 External R5 10,350.9 2514.3 800.9 601.2 59.2 33.1

R2 3306.0 818.8 269.2 −7795.6 −734.7 −400.5 R4 −1461.5 External −329.4 External −98.6External 3827.9External 358.6External 193.7 External R6 −1847.4 −489.2 −169.9 3939.6 372.7 204.7

As shown in Table4, as the fault resistance increased, the absolute values of the average products decreased to a certain extent, both for the internal faults and external faults. Nevertheless, the identiﬁcation results were all correct.

4.4.3. Different Fault Types

For the different fault types at f 4 on busbar II, or at f 3 on l3, the simulation data and identiﬁcation results are shown in Tables6 and7, respectively. Energies 2018, 11, 1139 12 of 16

Table 6. Simulation results of the different fault types for the busbar II faults.

Bg Cg ABg ABCg AB AC Unit S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result

R1 6167.3 1180.2 4004.9 7178.4 2325.5 1256.3 R3 1760.9External 431.7External 1173.4External 2189.4External 600.1External 232.8 External R5 −7935.3 −1598.3 −5176.3 −9334.9 −2925.3 −1505.9

R2 39,181.7 7819.1 25,531.1 46,328.2 14,427.4 7601.4 R4 28,278.0Internal 5743.8Internal 18,468.6Internal 33,655.6Internal 10,343.6Internal 5340.0 Internal R6 27,595.8 5381.1 17,929.2 32,573.2 10,193.4 5574.7

Table 7. Simulation results of the different fault types for the external faults.

Ag Bg ACg BCg AC BC Unit S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result S (kVA) Result

R1 318.9 1190.9 18,944.7 17,359.6 20,969.9 18,235.9 R3 −467.5 External −1746.9 External −28,122.1 External −25,727.3 External −31,130.8 External −27,016.0 External R5 147.3 549.7 9080.6 8278.6 10,057.5 8689.6 R2 213.6 786.2 12,540.1 11,431.9 13,886.4 11,997.6 R4 −284.4 External −1074.5 External −17,103.1 External −15,728.8 External −18,923.7 External −16,533.4 External R6 69.9 283.1 4484.1 4223.7 4953.6 4461.8

The simulation data shown in Tables6 and7 indicates that the method could identify the faults with the different fault types exactly.

4.5. Other Inﬂuence Factors

4.5.1. Series Capacitor Compensation In order to improve the EHV/UHV (Extra High Voltage/Ultra High Voltage) transmission capacity, series capacitors were generally installed in the long transmission lines. Therefore, it was essential to study the inﬂuence of the series capacitor compensation on the proposed technique. A series capacitor with a compensation coefﬁcient of 40 percent was installed at the start of l1, in Figure7. The different faults were set at f 3, f 4, and f 5 and the simulation data is indicated in Table8.

Table 8. Simulation data for the series compensation.

f 3 f 4 f 5 Unit S (kVA) Result S (kVA) Result S (kVA) Result

R1 10,494.1 1254.9 −1205.6 R3 −15,591.1 External 305.3External 786.2 External R5 5044.0 −1571.6 423.6

R2 7007.6 7723.4 −439.6 R4 −9430.1External 5484.6Internal 284.8 External R6 2379.6 5490.3 160.2

As shown in Table8, whether the fault was on the compensated line or not, the proposed protection criterion could correctly discriminate all of these faults.

4.5.2. CT Saturation Generally speaking, the performance of the traditional busbar differential protection method is easily affected by CT saturation especially the serious CT saturation. Assuming a phase B grounding fault occurs at f 3 on l3 with the phase B current of CT3 seriously saturated to testify the effect of CT saturation on the proposed method. The waveforms of the phase B currents for CT1, CT3 and CT5 are illustrated in Figure 11, and the differential currents of busbar I are shown in Figure 12. Figure 13 shows the waveforms of the calculated average products of busbar I, and Table9 shows the corresponding simulation results. The instant of fault occurrence is 10 ms for Figures 11–13. Energies 2018, 11, 1139 13 of 16

Energies 2018, 11, x FOR PEER REVIEW 13 of 16 Energies 2018, 11, x FOR PEER REVIEW 13 of 16 Table 9. Simulation results for a phase B fault on l3. Table 9. Simulation results for a phase B fault on l3. Table 9. Simulation results for a phase B fault on l3. BusbarBusbar ProtectionProtection Unit Unit S S(kVA) (kVA) Fault Fault Direction Direction λλ IdentiﬁcationIdentification ResultResult Busbar Protection Unit S (kVA) Fault Direction λ Identification Result R1R1 248.5248.5 BackwardBackward R1 248.5 Backward I I R3R3 −371.6−371.6 ForwardForward 11 External FaultFault I R5R53 125.4−371.6125.4 BackwardBackwardForward 1 External Fault R5 125.4 Backward R2R2 167.0167.0 BackwardBackward 2 II II R4R4 −220.2−167.0220.2 ForwardBackwardForward 11 External FaultFault II R6R64 54.6−220.254.6 BackwardBackwardForward 1 External Fault R6 54.6 Backward 4 2 0 -2 -4 0 5 10 15 20 25 30 35 40 Time (ms) (a) Phase B current of CT 5 1

-5 0 5 10 15 20 25 30 35 40 Time (ms) (b) Phase B current of CT 3 4 2 0 -2 -4 0 5 10 15 20 25 30 35 40 Time (ms) (c) Phase B current of CT 5

Figure 11. The phase B currents of CT1, CT3, and CT5 for a phase B fault on l3, on the condition of Figure 11. The phase B currents of CT1, CT3, and CT5 for a phase B fault on l3, on the condition of phaseFigure B 11 saturation. The phase of CTB currents3. of CT1, CT3, and CT5 for a phase B fault on l3, on the condition of phasephase B saturation B saturation of CT of3 CT. 3. 2

(kA) 0 dA i -2 0 5 10 15 20 25 30 35 40 Time (ms) (a) Phase A differential current for busbar I 5

(kA) 0 dB i -5 0 5 10 15 20 25 30 35 40 Time (ms) (b) Phase B differential current for busbar I 2

(kA) 0 dC i -2 0 5 10 15 20 25 30 35 40 Time (ms) (c) Phase C differential current for busbar I

Figure 12. Differential current of busbar I, on the condition of the phase B saturation of CT3. FigureFigure 12. Differential 12. Differential current current of busbarof busbar I, onI, on the the condition condition of the phase phase B B saturation saturation of ofCT CT3. 3.

Energies 2018, 11, 1139 14 of 16 Energies 2018, 11, x FOR PEER REVIEW 14 of 16

FigureFigure 13. The 13.average The average products product thats that were were detected detected by by R 1R,R1, R3,3, and and RR55 forfor a a phase phase B Bfault fault on on l3 (phasel3 (phase B B saturation of CT3). saturation of CT3).

From Figure 11, it can be seen that the phase B current of l3 reached saturation after 5.16 ms of Fromthe post Figure-fault 11. M,eanwhile, it can be seenFigure that 12 the shows phase the B phase current B differential of l3 reached current saturation of busbar after I, which 5.16 ms of theincreased post-fault. from Meanwhile, 0 to 5756 FigureA. Consequently 12 shows, the the busbar phase B differential differential protection current of of busbar busbar I I, may which increasedmaloperate. from 0 to As 5756 shown A. Consequently,in Figure 13 and the Table busbar 10, it differential has been reve protectionaled that of the busbar CT saturation I may maloperate. had no As showninfluence in Figure on the 13 proposed and Table busbar 10, itprotection has been. revealedThis is because that the the CT identification saturation criterion had no inﬂuenceonly used on the proposed2.25 ms of busbar the data protection. window of This the post is because-fault, but the at identiﬁcationthat time the CT criterion saturation only did used not appear. 2.25 ms of the data window of the post-fault, but at that time the CT saturation did not appear. Table 10. Simulation results for a phase B fault on l3.

TypicalTable Internal 10. Simulation Fault resultsTypical for External a phase Fault B fault on l3. Fault at f1 Unit S (kVA) Result S (kVA) Result S (kVA) Result R1 Typical901.9 Internal Fault Typical−16,383.0 External Fault1339.5 Fault at f 1 Unit R3 S (kVA)797.9 ResultInternal S13,572.1(kVA) ResultExternal S (kVA)2366.5 ResultInternal

RR1 5 901.9577.9 −16,383.04400.4 1339.51689.4 RR3 2 797.9170.8 Internal 13,572.1−37,689.2 External 2366.51691.1 Internal R 577.9 4400.4 1689.4 R5 4 −88.4 External 19,409.3 External −783.8 External

RR2 6 170.8−82.8 −37,689.218,264.1 1691.1−916.9 R4 −88.4External 19,409.3 External −783.8 External R −82.8 18,264.1 −916.9 4.5.3. CVT 6Transfer Characteristic The CVT (Capacitor Voltage Transformer) is currently widely used in the EHV/UHV power 4.5.3. CVT Transfer Characteristic line, so the influence of the CVT transient transfer characteristic on the proposed algorithm could Thenot CVTbe neglected (Capacitor. Setting Voltage the same Transformer) fault conditions is currently as the widelytypical usedfaults, in in the Section EHV/UHV 4.3, and poweranother line, so thephase inﬂuence B ground of theing CVT fault transient at f1 with transfer the fault characteristic inception angle on ofthe 0°, proposed and then, algorithm Table 10 shows could not the be neglected.relevant Setting simulation the same data. fault conditions as the typical faults, in Section 4.3, and another phase As shown in Table 10, the proposed technique could identify◦ these faults correctly, after taking B grounding fault at f 1 with the fault inception angle of 0 , and then, Table 10 shows the relevant the CVT transient transfer characteristic into account. simulation data. As4.6 shown. Operation in TableSpeed Analysis10, the proposed technique could identify these faults correctly, after taking the CVT transient transfer characteristic into account. The operation time of the busbar protection device basically included the filtering delay, 4.6. Operationphase-mode Speed transformation, Analysis the calculation of the average product of aerial mode components, and the logical judgment, so it consumed little time. In view of the sampling frequency of 4 kHz, 10 Theconsecutive operation samples, time and of thethe data busbar window protection of 2.25 ms, device the total basically time that included used to thedetect ﬁltering a fault did delay, phase-modenot exceed transformation, 5 ms. Therefore, the the calculation novel busbar of the protection average had product a rapid of aerialoperation mode speed. components, and the logical judgment, so it consumed little time. In view of the sampling frequency of 4 kHz, 10 consecutive samples, and the data window of 2.25 ms, the total time that used to detect a fault did not exceed 5 ms. Therefore, the novel busbar protection had a rapid operation speed. Energies 2018, 11, 1139 15 of 16

5. Conclusions This paper proposes a fast busbar protection method, based on the characteristic of the average product of the aerial mode components. Referring to an actual 500 kV substation, a simulation model was built and massive simulations were performed. From the theoretical analyses and simulation data, the following conclusions could be drawn: 1. The busbar protection technique is applicable to both a single busbar conﬁguration and one and a half breakers busbar conﬁguration. Furthermore, the sampling frequency is set as 4 kHz, which is in accordance with the standard sampling frequency of the merging unit in the smart substation. Therefore, the method has a good adaptability. 2. The performance of the proposed technique is hardly affected by the fault initial conditions, CT saturation, series compensation, and CVT transfer characteristic. Hence, the method has high reliability and sensitivity. 3. The proposed technique has a rapid operation speed, and the operation time is usually less than 5 ms.

Author Contributions: Authors have worked on this manuscript together. Acknowledgments: This work was supported in part by National Key Research and Development Program of China (2016YFB0900603), and part by National Nature Science Foundation of China (51277114). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

CT current transformer PT potential transformer CVT capacitor voltage transformer B busbar CB circuit breaker l line R detection unit of protection ∆u aerial mode voltage ∆i aerial mode current S average product

References

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