Insulation Reconstruction for OPGW DC De-Icing and Its Influence on Lightning Protection and Energy Conservation

energies

Article Insulation Reconstruction for OPGW DC De-Icing and Its Inﬂuence on Lightning Protection and Energy Conservation

Xiangxin Li 1,2,*, Ming Zhou 1, Yazhou Luo 2, Chao Xia 3, Bin Cao 4 and Xiujuan Chen 3 1 State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Changping District, Beijing 102206, China; [email protected] 2 National Power Grid Corp North China Branch, Xicheng District, Beijing 100053, China; [email protected] 3 China Electric Power Research Institute, Haidian District, Beijing 100000, China; [email protected] (C.X.); [email protected] (X.C.) 4 Inner Mongolia Electric Power Research Institute, Hohhot 010020, China; [email protected] * Correspondence: [email protected]; Tel.: +86-010-8358-2678

Received: 12 August 2018; Accepted: 3 September 2018; Published: 14 September 2018

Abstract: In order to satisfy demands for DC de-icing of optical fiber composite overhead ground wire (OPGW) and solve questions such as those relating to circulating current loss and liability of suffering from lightning strike, the grounding method of OPGW must be changed from the current commonly used method of being grounded at every tower to being grounded at one tower. The OPGW would be connected to the tower by an insulator, which is often shunt connected with a protective discharge clearance. The recommended value of the discharge clearance is from 70 to 80 mm. The lightning impulse discharge voltage of such a clearance is generally not more than 100 kV. However, as the transmission line is struck by lightning, over-voltage on the clearance is 885 kV at least, even up to a few MV. The clearance can be broken down reliably. The influence of insulation reconstruction for OPGW on the induced current and the power loss of the AC transmission line was studied by means of theoretical analysis and simulation calculations. Results indicate that change of the OPGW grounding mode could reduce the induced current of the ground wire to below 1 A and reduce the power loss of the line to below 1 W/km. Power loss could be reduced by over 99%. Adoption of a suitable grounding mode for OPGW is of great significance for DC de-icing, lightning protection safety, and energy savings for UHV projects.

Keywords: optical ﬁber composite overhead ground wire; grounded at every tower; grounded at one tower; ground wire insulator; discharge clearance; DC de-icing; insulation reconstruction

1. Introduction With the technical requirements of lightning protection, communication, energy conservation, and DC de-icing considered, the grounding mode of optical ﬁber composite overhead ground wire (OPGW) usually is of the type of being grounded at a single point, being grounded at every tower, or being grounded at one tower. The type used should be determined by a comparison of technology and economy. At present, OPGW is generally grounded at every tower, yet common ground wire mainly uses graded insulation and being grounded at a single point. There is a degree of induction voltage, electrostatic induction current, and electromagnetic induction current on ground wire because of electrostatic coupling and electromagnetic induction between ground wire and the transmission line; in that case, circulating current loss happens in OPGW, which negatively affects the diminishing line loss and energy economy.

Energies 2018, 11, 2441; doi:10.3390/en11092441 www.mdpi.com/journal/energies Energies 2018, 11, 2441 2 of 11

On the other hand, the difference in the grounding method between OPGW and common ground wire in the same tower increases the possibility of lightning strikes on OPGW and the problem of breaking stock [1–4]. So, it is necessary to improve the OPGW’s grounding mode. In order to reduce the electromagnetic induction current and power loss caused by the overhead ground wire being grounded at every tower, it is appropriate to adopt the grounding mode of single-point grounding. The earth point could be set on the top or the middle of the overhead ground wire. In order to satisfy the need for DC de-icing of OPGW in winter, it is necessary to do an insulation reconstruction of the OPGW. Since the 2008 mass ice disaster, State Grid Corporation of China (SGCC) and China Southern Power Grid (CSG )have performed insulation reconstruction for the purpose of DC de-icing of the OPGW of multiple 110~500 kV AC transmission lines [5–7]. After reconstruction, the OPGW is connected with the tower by a ground wire insulator which should be equipped with a parallel discharge gap. The choice of gap distance is key of the application of the parallel gap. However, related articles [8–20] do not confirm the method of choosing or the requirements of the parameters. Electrical requirements and technical conditions of the parallel discharge gap need to be studied further in order to confirm a suitable gap distance. With large-scale construction of UHV AC transmission lines and large-scale adoption of OPGW in this construction, ground wire loss per kilometer will be remarkable; this is bad for energy saving if the grounding mode of the OPGW is not reasonable. The lightning trip-out rate of a UHV AC transmission line has a relatively high requirement, so it must be limited to under 0.1 times per hundred kilometers per year (translating into 40 thunderstorms per day) [21]. The growing demand for wires and ground wire de-icing is more and more important since the 2008 mass ice disaster. Therefore, according to power loss, anti-lightning performance, and de-icing demand, we should choose a suitable grounding mode for OPGW in UHV AC transmission lines [19–23]. This paper first gives the electrical requirement of the parallel discharge gap for OPGW DC de-icing. At normal operation, under the DC de-icing condition, and under the condition of lightning, the OPWG’s ground insulator electrical characteristics of the parallel discharge gap are analyzed. The recommended values are given in line with the requirements of the parallel discharge gap distance. Then, taking the north Zhejiang to Fuzhou UHV AC transmission line as an example, we combine theoretical analysis and simulation computations to research the influence of AC transmission line power loss and line lightning protection performance on different grounding modes of OPGW. Finally, the optimum grounding mode of OPGW is given.

2. Common De-Icing Connection Modes and Requirements of OPGW and Common Ground Wire

2.1. De-Icing Connection Modes In Connection Mode 1, the de-icing mode of OPGW and common ground wire form a loop circuit by lead lines. If the two ground wires are an OPGW and a common ground wire, generally we do de-icing by Connection Mode 1. This connection mode is shown in Figure1a. Connection Mode 2 is a parallel multiple de-icing mode. In the parallel de-icing mode, the OPGW has been repeated parallel with the common ground wire, and forms a loop circuit by way of a quarter-phase line. This connection mode is shown in Figure1b. In Connection Mode 3, the OPGW has a parallel with the common ground wire and then connects in series with two circuits. This program is mainly aimed at single and double hybrid lines. This de-icing mode is shown in Figure1c. Energies 2018, 11, 2441 3 of 11 Energies 2018, 11, x FOR PEER REVIEW 3 of 11

De-icing mode of OPGW De-icing mode of common earth wire Positive wire Positive wire Common earth wire Common earth wire OPGW OPGW

Negative wire Negative wire (a) De-icing mode of OPGW De-icing mode of common earth wire Positive wire Positive wire Common earth wire Common earth wire OPGW OPGW

Negative wire Negative wire (b) Positive wire Common earth wire OPGW

Negative wire (c)

FigureFigure 1. DC1. DC de-icing de-icing circuit: circuit: ( a(a)) Connection Connection Mode 1; 1; ( (bb) )Connection Connection Mode Mode 2; ( 2;c) (Connectionc) Connection Mode Mode 3. 3.

2.2.2.2. De-Icing De-Icing Requirements Requirements WhetherWhether the the OPGW OPGW de-icing de-icing programs programs use series or or parallel parallel connections, connections, the the OPGW OPGW must must afford afford a rangea range of de-icingof de-icing voltages. voltages. The The peak peak voltagevoltage of OPGW is is generally generally ±20± 20kV, kV, and and voltage voltage distribution distribution on theon the OPWG OPWG from from the the power power end end to the to groundingthe groundin endg end is 0~ is± 0~±2020 kV, kV, so theso groundingthe grounding mode mode of OPGW of mustOPGW be changed must be from changed being from grounded being grounded at every towerat every to tower single-point to single-point grounding. grounding. The earth The pointearth can be setpoint on can the be top set or on the the middle top or the of themiddle overhead of the groundoverhead wire. ground wire.

3. Electrical3. Electrical Requirements Requirements and and Gap Gap DistanceDistance Selection for for Parallel Parallel Discharge Discharge Gap Gap on onOPGW OPGW DC DC De-IcingDe-Icing Insulation Insulation Transformation Transformation

3.1.3.1. Structure Structure of Groundof Ground Wire Wire Insulator Insulatorand and ParallelParallel Discharge Gap Gap TheThe structures structures of of the the ground ground wire wire insulator insulator and and parallel parallel gap gap are are shown shown in in Figure Figure2 .2. In In Figure Figure2a,b 2a,b are shown the pendant and tension structure charts of the ground wire insulator and parallel are shown the pendant and tension structure charts of the ground wire insulator and parallel discharge discharge gap, respectively. The technical parameters of the ground wire insulator are shown in Table gap, respectively. The technical parameters of the ground wire insulator are shown in Table1. 1. The parallel gap is composed of two electric poles which are installed on fittings; one pole is fixed The parallel gap is composed of two electric poles which are installed on ﬁttings; one pole is ﬁxed on on the tower and the other is connected with the pre-twisted suspension clamp of OPGW. The thedistance tower and of the otherelectric is poles connected can be with adjusted the pre-twisted by voltage suspensionlevel, and the clamp guide of OPGW.arc poles The which distance are of theinstalled electric poleson the can OPGW be adjusted are made by of voltageheat-resistant level, steel. and the guide arc poles which are installed on the OPGW are made of heat-resistant steel.

Table 1. Technical parameters of the ground wire insulator.

Parameters Value Rated mechanical load [kN] 100 Tensile test load [kN] 50 Structural height [mm] 328 Minimal arc distance [mm] ≥158 Nominal climbing distance [mm] 490 Diameter of mandrel [mm] 18 Diameter of large/small umbrella(a skirt) [mm] 134(large)/90(small) Parachute spacing [mm] ≥80 Sign of connecting structure 16 N Thickness of upper electrode [mm] ≥2.5 Diameter of lower electrode [mm] ≥12 Adjustment range of gap [mm] 0~500 Energies 2018, 11, x FOR PEER REVIEW 3 of 11

De-icing mode of OPGW De-icing mode of common earth wire Positive wire Positive wire Common earth wire Common earth wire OPGW OPGW

Negative wire Negative wire (a) De-icing mode of OPGW De-icing mode of common earth wire Positive wire Positive wire Common earth wire Common earth wire OPGW OPGW

Negative wire Negative wire (b) Positive wire Common earth wire OPGW

Negative wire (c)

Figure 1. DC de-icing circuit: (a) Connection Mode 1; (b) Connection Mode 2; (c) Connection Mode 3.

2.2. De-Icing Requirements Whether the OPGW de-icing programs use series or parallel connections, the OPGW must afford a range of de-icing voltages. The peak voltage of OPGW is generally ±20 kV, and voltage distribution on the OPWG from the power end to the grounding end is 0~±20 kV, so the grounding mode of OPGW must be changed from being grounded at every tower to single-point grounding. The earth point can be set on the top or the middle of the overhead ground wire.

3. Electrical Requirements and Gap Distance Selection for Parallel Discharge Gap on OPGW DC De-Icing Insulation Transformation

3.1. Structure of Ground Wire Insulator and Parallel Discharge Gap The structures of the ground wire insulator and parallel gap are shown in Figure 2. In Figure 2a,b are shown the pendant and tension structure charts of the ground wire insulator and parallel discharge gap, respectively. The technical parameters of the ground wire insulator are shown in Table 1. The parallel gap is composed of two electric poles which are installed on fittings; one pole is fixed on the tower and the other is connected with the pre-twisted suspension clamp of OPGW. The distance of the electric poles can be adjusted by voltage level, and the guide arc poles which are Energies 2018, 11, 2441 4 of 11 installed on the OPGW are made of heat-resistant steel.

Energies 2018, 11, x FOR PEER REVIEW 4 of 11 (a)

(b)

Figure 2.Figure Structure 2. Structure diagrams diagrams of of the ground ground wire wire insulator insulator and parallel and discharge parallel gap: discharge (a) the pendants; gap: (a) the pendants;(b )(b the) the tensioning. tensioning. 3.2. Electrical Requirements for Ground Wire Insulator and Parallel Discharge Gap in OPGW DC De-Icing Insulation TransformationTable 1. Technical parameters of the ground wire insulator.

The electrical requirementsParameters for the OPGW DC de-icing insulation reconstructionValue of the ground wire insulator and parallelRated discharge mechanical gap load are as [kN] follows: 100 (1) The ground wire insulatorTensile test and load parallel [kN] gap should not be broken under 50 the influence of line-induced voltage,Structural and power height frequency [mm] discharge voltage with the parallel 328 gap should be less than the powerMinimal frequency arc distance withstand [mm] voltage without the gap. ≥158 Nominal climbing distance [mm] 490 (2) The ground wire insulator and parallel gap should not be broken under the influence of the DC Diameter of mandrel [mm] 18 de-icing voltage. Diameter of large/small umbrella skirt [mm] 134(large)/90(small) (3) The parallel gap of theParachute ground spacing wire insulator [mm] should break under the influence≥80 of lightning in order to protect theSign insulator. of connecting structure 16 N Thickness of upper electrode [mm] ≥2.5 3.3. Requirements of Parallel Gap Power Frequency Discharge Voltage and the Selection of Gap Distance for the Parallel Discharge GapDiameter of lower electrode [mm] ≥12 Adjustment range of gap [mm] 0~500 (1) Requirements of Parallel Gap Power Frequency Discharge Voltage 3.2. ElectricalIt Requirements is known from for the Ground above calculation Wire Insula resultstor and that Parallel the induced Discharge voltage Gap of the in groundOPGW wire DC isDe-Icing Insulationlimited Transformation to under 1 kV; that is, the maximum power frequency voltage of the ground wire insulator and parallel gap is under 1 kV. If the safety factor is 1.15, valid values of the power frequency withstand Thevoltage electrical between requirements the ground for wire the insulator OPGW and DC parallel de-icing gap shouldinsulation be above reconstr 1.15 kV.uction At the of same the ground wire insulatortime, the and parallel parallel gap shoulddischarge also meetgap are the requirementsas follows: of insulation of the ground wire insulator, which means that the power frequency discharge voltage of the parallel gap should be less than the (1) Thepower ground frequency wire insulator withstand voltageand parallel of the ground gap should wire insulator not be without broken a parallel under gap. the influence of line- induced voltage, and power frequency discharge voltage with the parallel gap should be less than the power frequency withstand voltage without the gap. (2) The ground wire insulator and parallel gap should not be broken under the influence of the DC de-icing voltage. (3) The parallel gap of the ground wire insulator should break under the influence of lightning in order to protect the insulator.

3.3. Requirements of Parallel Gap Power Frequency Discharge Voltage and the Selection of Gap Distance for the Parallel Discharge Gap (1) Requirements of Parallel Gap Power Frequency Discharge Voltage It is known from the above calculation results that the induced voltage of the ground wire is limited to under 1 kV; that is, the maximum power frequency voltage of the ground wire insulator and parallel gap is under 1 kV. If the safety factor is 1.15, valid values of the power frequency withstand voltage between the ground wire insulator and parallel gap should be above 1.15 kV. At the same time, the parallel gap should also meet the requirements of insulation of the ground wire insulator, which means that the power frequency discharge voltage of the parallel gap should be less than the power frequency withstand voltage of the ground wire insulator without a parallel gap.

Energies 2018, 11, 2441 5 of 11

Energies 2018, 11, x FOR PEER REVIEW 5 of 11 The DXF—160 CN is a pendant insulator and the DXF—100 C is a tension insulator which cannot be brokenThe DXF—160 in 75 kV. TheCN is safety a pendant factor insulator is 1.15, so and the the power DXF—100 frequency C is a discharge tension insulator voltage ofwhich the parallel cannot gapbe broken should in be 75 under kV. The 75 kVsafety÷ 1.15 factor = 65is 1.15, kV. so the power frequency discharge voltage of the parallel (2)gap should Testing be of under Power 75 Frequency kV ÷ 1.15 Discharge= 65 kV. Voltage and Withstand Voltage of the Parallel Gap (2) ImagesTesting ofof thePower setup Frequency for power Discharge frequency Voltage discharge and voltage Withstand tests Voltage of the ground of the wireParallel insulator Gap and parallelImages gap inof thetheUHV setup DC for base power of SGCCfrequency are shown discharge in Figure voltage3. From tests suchof the pictures, ground the wire situation insulator of theand testparallel site can gap be in seen, the includingUHV DC power,base of transformer, SGCC are shown and test in products. Figure 3. The From results such of pictures, the tests arethe shownsituation in of Table the2 test. As site shown can be in Tableseen,2 including, the power power, frequency transformer, discharge and voltages test products. (effective The values) results all of fallthe tests in the are range shown of 1.15~65 in Table kV 2. As when shown the distancein Table 2, of the the power parallel frequency gap is 20~100 discharge mm. voltages (effective values) all fall in the range of 1.15~65 kV when the distance of the parallel gap is 20~100 mm. Table 2. Test results of the power frequency discharge voltage of ground wire insulator dischargeTable 2. Test clearance. results of the power frequency discharge voltage of ground wire insulator discharge clearance. Power Frequency Discharge Voltage (Valid Values) [kV] Distance of the Parallel Gap [mm] Power Frequency Discharge Voltage (Valid Values) [kV] Distance of the Parallel Gap [mm] Pendant Insulator Tension Insulator Pendant Insulator Tension Insulator 20 29 28 20 29 28 40 39 44 40 39 44 60 45 52 60 45 52 100100 62 53 53

Figure 3. Pictures of power frequency discharge voltage testing in the UHV DC base of SGCC.

4. Electrical Requirement of OPGW Ground Wire Insulator and Parallel GapGap inin thethe CaseCase ofof Ground Wire DC De-Icing Ground Wire DC De-Icing 4.1. Requirements and Conditions of Icing Withstand Voltage Testing 4.1. Requirements and Conditions of Icing Withstand Voltage Testing The icing withstand voltage of the ground wire insulator and parallel gap should be higher than The icing withstand voltage of the ground wire insulator and parallel gap should be higher than the maximum DC de-icing voltage that ground wire DC de-icing needs. Icing experiments have been the maximum DC de-icing voltage that ground wire DC de-icing needs. Icing experiments have been made [8–13]. The following test conditions were formulated. made [8–13].The following test conditions were formulated. (1) RatedRated voltage voltage was was −−2020 kV; kV; (2) TheThe structure structure of the insulator is shown in Figure1 1.. IcingIcing withstandwithstand voltagevoltage testtest picturespictures areare shownshown in in Figure Figure 44.. Tests werewere carriedcarried outout inin thethe UHVUHV DCDC basebase ofof SGCC;SGCC; (3) TheThe tested tested distances distances of of the the parallel parallel gap were 60 mm, 80 mm, and 100 mm; (4) IcingIcing thickness thickness tested tested were were 20 20 mm mm and and 30 mm (heavy icing area); (5) SaltSalt deposit deposit density density 0.08 0.08 mg/cm mg/cm2,2 non-soluble, non-soluble deposi depositt density density 1.0 1.0 mg/cm mg/cm2; 2; (6) TheThe pressure pressure method used was the booster method.

Energies 2018, 11, 2441 6 of 11 Energies 2018, 11, x FOR PEER REVIEW 6 of 11

Figure 4. IcingIcing withstand withstand voltage voltage test test pictures pictures in in the UHV DC base of SGCC.

4.2. Test Test Results The icing icing test test results results of of the the ground ground wire wire insula insulatortor and and parallel parallel discharge discharge clearance clearance are are shown shown in Tablein Table 3. 3. Table 3. Icing test results of the ground wire insulator and parallel discharge clearance. Table 3. Icing test results of the ground wire insulator and parallel discharge clearance. Salt Deposit Density Salt Deposit Density Icing Thickness Parallel Gap Icing Withstand Insulator (Non-Soluble Deposit Icing Thickness Parallel Gap Icing Withstand Insulator (Non-Soluble Deposit2 [mm] [mm] Voltage [kV] Density) [mg·cm ] [mm] [mm] Voltage [kV] Density) [mg·cm2] 0.08 (1.0) 20 60 −23 0.08 (1.0) 20 60 −23 Pendant insulator 0.08 (1.0) 30 80 −24 Pendant insulator 0.080.08 (1.0) (1.0) 30 30 100 80 −−2425 0.08 (1.0) 30 100 −25 Tension insulator 0.08 (1.0) 20 60 −23 Tension insulator 0.08 (1.0) 20 60 −23

From Table 3 3,, somesome conclusionsconclusions can can be be made. made. The The DC DC de-icing de-icing voltage voltage is is −2020 kV. kV. In In consideration consideration of overthrow about ±±10%,10%, the the highest highest DC DC de-icing de-icing voltage reached is −2222 kV. kV. The icing withstand voltage ofof thethe groundground wire wire insulator insulator and and parallel parallel gap gap should should be higherbe higher than than the maximumthe maximum DC de-icingDC de- icingvoltage voltage of −22 ofkV −22 that kV thethat ground the ground wire wire DC de-icing DC de-icing needs. needs. If the If maximum the maximum icing-thickness icing-thickness is 30 is mm, 30 mm,the gap the distance gap distance should should be above be 60 above mm. In60 considerationmm. In consideration of the dispensability of the dispensability of gap discharging, of gap discharging,the gap distance the gap should distance be larger, should so 70~80be larger, mm so is suggested.70~80 mm is suggested.

5. The Inﬂuence of UHV AC Line Lightning Protection Performance on Different Grounding 5. The Influence of UHV AC Line Lightning Protection Performance on Different Grounding Modes of OPGW Modes of OPGW 5.1. The Requirement of a Ground Wire OPGW Insulator and Parallel Gap in the Case of Lightning 5.1. The Requirement of a Ground Wire OPGW Insulator and Parallel Gap in the Case of Lightning In the case of lightning, the lightning impact discharge voltage of the parallel gap should be less thanIn the the lightning case of lightning, impact withstand the lightning voltage impact of the discharge ground wirevoltage insulator. of the parallel That is gap to say, should the parallel be less thangap shouldthe lightning be broken impact reliably withstand to protect voltage the groundof the gr wireound insulator wire insulator. in the case That of is lightning. to say, the Generally, parallel gapa ratio should of minimal be broken arc reliably distance to of protect the parallel the ground gap and wire ground insulator wire in insulatorthe case of between lightning. 80% Generally, and 85% acan ratio meet of minimal the requirements. arc distance When of the the parallel minimal gap arc and distance ground of thewire parallel insulator gap between is more 80% than and 158 85% mm canand meet the parallel the requirements. gap distance When is between the minimal 126.4 mm arc and distance 134.3 of mm, the the parallel parallel gap gap is ismore broken than before 158 mm the andground the wireparallel insulator. gap distance In this paper,is between if the 126.4 parallel mm gap and distance 134.3 mm, is 70 the mm parallel and 80 gap mm, is the broken requirement before thethat ground the parallel wire gap insulator. is broken In before this paper, the ground if the wire parallel insulator gap distance can be met. is 70 mm and 80 mm, the requirement that the parallel gap is broken before the ground wire insulator can be met. 5.2. Lightning Over-Voltage Calculation of a 220 kV Ground Wire Insulator and Parallel Gap 5.2. Lightning Over-Voltage Calculation of a 220 kV Ground Wire Insulator and Parallel Gap There are two lightning over-voltages on a 220 kV ground wire insulator and parallel gap separatelyThere causedare two by lightning shield failureover-voltages and back on striking a 220 kV [14 –ground19]. These wire two insulator lightning and over-voltages parallel gap separatelycan be calculated caused by by the shield Electro-Magnetic failure and back Transient striking Program [14–19]. (EMTP). These two The lightning simulation over-voltages model is shown can bein Figurecalculated5. by the Electro-Magnetic Transient Program (EMTP). The simulation model is shown in Figure 5.

Energies 2018,, 11,, 2441x FOR PEER REVIEW 77 of of 11 11

Figure 5. 5. LightningLightning over-voltage over-voltage simulation simulation model. model.

From this simulation model, the following results were attained: (1)(1) BackBack striking: striking: According According to to GB/T GB/T 50064-2014 50064-2014 [20], [20], the the back back striking striking lightning lightning current current of of a a 220 220 kV kV ACAC transmission transmission line line is is 75~110 75~110 kA. kA. In In the the simulati simulation,on, if the if the back back striking striking lightning lightning current current is 75 is kA,75 kA, then then the themaximum maximum voltage voltage on the on theparallel parallel gap gap is 13 is MV. 13 MV. (2)(2) ShieldShield failure: failure: Shield Shield failure failure of of the the 220 220 kV kV tower tower is is 16 16 kA, kA, and and the the maximum maximum voltage voltage on on the the parallelparallel gap gap is is 885 885 kV kV in in the the simulation. simulation.

5.3. Lightning Over-Voltage Calculation of of a a 500 500 kV kV Ground Ground Wire Wire Insulator Insulator and and Parallel Parallel Gap Gap There are also two lightning over-voltages on on a a 500 500 kV kV ground ground wire wire insulator insulator and and parallel parallel gap gap separately caused by shield fail failureure and back striking. From From a a simulation simulation model model similar similar to to that that in in Figure5 5,, thethe followingfollowing resultsresults werewere obtained:obtained:

(1)(1) BackBack striking: striking: According According to to GB/T GB/T 50064-2014 50064-2014 [20], [20], the the back back striking striking lightning lightning current current of of a a 500 500 kV kV ACAC transmission line line is is 125~175 125~175 kA. kA. In In the the simulation, simulation, if if the the back back striking striking lightning lightning current current is is 125125 kA, kA, the the maximum maximum voltage voltage on on the the parallel parallel gap gap is is 15 15 MV. MV. (2)(2) ShieldShield failure: failure: Similarly, Similarly, the the shield shield failure failure of of a a 500 500 kV kV tower tower is is 25 25 kA, kA, and and the the maximum maximum voltage voltage onon the the parallel parallel gap gap is is 1.36 1.36 MV MV in in the the simulation. simulation.

5.4. Capability Check of Parallel Gap in in the the Case Case of of Lightning Lightning In the case of of lightning, lightning, the the gr groundound wire wire insulator insulator and and parallel parallel gap gap should should meet meet the the requirements requirements of reliablereliable breakdown.breakdown. It It is is known known from from the the above above calculation calculation results results that thethat voltage the voltage which which the ground the wireground insulator wire insulator and parallel and gapparallel can affordgap can is afford 885 kV is at 88 least,5 kV evenat least, up toeven several up to MV several in the MV case in ofthe 220 case kV andof 220 500 kV kV and AC 500 transmission kV AC transmission lines suffering lines sufferi shieldng failure shield and failure back and striking. back striking. The lightning The lightning impact dischargeimpact discharge voltage voltage of a gap of distance a gap distance between between 70 mm and 70 80mm mm and is 80 under mm 100is under kV generally. 100 kV generally. Under this highUnder lightning this high impulse lightning voltage, impulse a gapvoltage, distance a gap between distance 70 between mm and 70 80mm mm and must 80 mm be broken.must be broken.

5.5. Simulation Method and Calculat Calculationion Results Results of of the the Effects Effects of of Graded Graded Insulation Insulation of of OPGW OPGW on on Lightning Lightning Protection Performance This paper calculates the lightning trip-out trip-out rate rate of of a a UHV UHV AC AC transmission transmission line line when when OPGW OPGW uses different grounding modes modes and and compares compares the the resu results.lts. Results Results are are shown shown in in Table Table 4.4. In In Table Table 44,, Approach 1 is OPGW being ground groundeded at at every every tower, tower, and and Approach Approach 2 2 is is OPGW OPGW being being grounded grounded at at one tower. As shown in Table 4 4,, therethere isis nono effecteffect whenwhen we change the grounding mode of of OPGW, OPGW, because because the parallel gap of the insulator is mainly broken in the the case case of of lightning lightning impulse impulse voltage. voltage.

Energies 2018, 11, 2441 8 of 11

Table 4. Inﬂuence of different grounding modes of OPGW on lightning trip-out rate of a UHV AC transmission line.

Approach 1 Approach 2 Thunderstorm Days Lightning Trip-Out Rate Lightning Trip-Out Rate [Time/100 km·Year] [Time/100 km·Year] 90 0.263 0.263 40 0.092 0.092

6. The Inﬂuence of Grounding Modes of OPGW on Power Loss of a UHV AC Transmission Line

6.1. Calculation Method of Power Loss The power loss of the ground wire was calculated using the Electro-Magnetic Transient Program Energies(EMTP) 2018, 11 based, x FOR on PEER a simulation; REVIEW the calculation method is shown in Figure6. 9 of 11

FigureFigure 6. Calculation 6. Calculation method method of of electric energyenergy loss. loss.

There is a relationship between the power loss of the ground wire and the transmission power of the line. Such a relationship is shown in Table 6 and Figure 7. When the transmission power is the same, the power loss from being grounded at every tower is much larger than that from graded insulation. When OPGW uses graded insulation, power loss can be reduced by over 99% when compared with another type as shown in Table 6.

Table 6. Power loss on a UHV AC overhead line.

Power Loss on Ground Wire [kW/km] Load Flow [MW + jMvar] Approach 1 Approach 2 2053 − j73 0.493 0.0007 3030 − j100 1.073 0.0007 4018 − j127 1.875 0.0007 5034 − j133 2.957 0.0007 6012 − j179 4.215 0.0007

Energies 2018, 11, 2441 9 of 11

6.2. Results of Power Loss Take the UHV AC from north Zhejiang to Fuzhou as an example. The results of induced current on the UHV AC overhead ground wire are shown in Table5. As shown in Table5, the induced current for OPGW grounded at every tower is much larger than that for OPGW with graded insulation and single-point grounding.

Table 5. Induced current on UHV AC overhead ground wire.

Induced Current on Ground Wire [A] Load Flow [MW + jMvar] Approach 1 Approach 2 OPGW Common Ground Wire OPGW Common Ground Wire 2054 − j74 30.42 0.75 0.81 0.75 3032 − j102 44.88 0.75 0.81 0.75 4019 − j128 59.32 0.75 0.81 0.75 5035 − j134 74.49 0.75 0.81 0.75 6014 − j180 88.95 0.75 0.81 0.75

There is a relationship between the power loss of the ground wire and the transmission power of the line. Such a relationship is shown in Table6 and Figure7. When the transmission power is the same, the power loss from being grounded at every tower is much larger than that from graded insulation. When OPGW uses graded insulation, power loss can be reduced by over 99% when compared with another type as shown in Table6.

Table 6. Power loss on a UHV AC overhead line.

Figure 7. Relationship between power loss of ground wire andand lineline powerpower ﬂow.flow. 7. Conclusions 7. Conclusions In order to satisfy the demands of DC de-icing of an optical ﬁber composite overhead ground In order to satisfy the demands of DC de-icing of an optical fiber composite overhead ground wire (OPGW) and solve questions such as those relating to circulating current loss and the liability of wire (OPGW) and solve questions such as those relating to circulating current loss and the liability suffering from lightning stroke and breaking, the grounding method of OPGW must be changed from of suffering from lightning stroke and breaking, the grounding method of OPGW must be changed the current commonly used method of being grounded at every tower to being grounded at one tower. from the current commonly used method of being grounded at every tower to being grounded at one tower. OPGW would be insulation reconstructed and be connected with the tower by a ground wire insulator, which should be equipped with a protective discharge clearance. The recommended value of this discharge clearance is from 70 to 80 mm, which was calculated and experimentally researched. The lightning impulse discharge voltage of such a 70 to 80 mm clearance is generally not more than 100 kV. However, as the transmission line is struck by lightning, over-voltage on the insulator and clearance is 885 kV at least, even up to a few MV. The clearance can be broken down reliably to ensure the safety of the insulator. Changing the grounding mode of OPGW has no effect on lightning performance. As the grounding mode of OPGW is changed from being grounded at every tower to graded insulation and being grounded at one tower, the induced current of the ground wire can be reduced to below 1 A and power loss can be reduced to below 1 W/km, a reduction of over 99%.

Acknowledgments: This research work was supported by the Science and Technology Project of SGCC (GYB17201600174). The authors are grateful for this support.

Conflicts of Interest: The authors declare no conflict of interest.

References

Energies 2018, 11, 2441 10 of 11

OPGW would be insulation reconstructed and be connected with the tower by a ground wire insulator, which should be equipped with a protective discharge clearance. The recommended value of this discharge clearance is from 70 to 80 mm, which was calculated and experimentally researched. The lightning impulse discharge voltage of such a 70 to 80 mm clearance is generally not more than 100 kV. However, as the transmission line is struck by lightning, over-voltage on the insulator and clearance is 885 kV at least, even up to a few MV. The clearance can be broken down reliably to ensure the safety of the insulator. Changing the grounding mode of OPGW has no effect on lightning performance. As the grounding mode of OPGW is changed from being grounded at every tower to graded insulation and being grounded at one tower, the induced current of the ground wire can be reduced to below 1 A and power loss can be reduced to below 1 W/km, a reduction of over 99%.

Author Contributions: X.L. and M.Z. conceived and designed the experiments; C.X. performed the experiments; X.C., B.C. and Y.L. analyzed the data; X.C. contributed analysis tools; X.L. wrote the paper. Acknowledgments: This research work was supported by the Science and Technology Project of SGCC (GYB17201600174). The authors are grateful for this support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Xi, C.; Qi, L.; Chen, X.; Zhu, H. Research on Operation Safety of OPGW Grounding Mode; Electric Power Research Institute: Beijing, China, 2014; pp. 3–48. 2. Li, Y.; Zeng, M.L.; Wang, D.; Liu, Q. Analysis and Research of Grounding Modes of 750 kV Transmission Line. Technol. Appl. 2015, 94, 82–84. 3. Xi, C.; Qi, L.; Chen, X.; Zhu, H. Research on the Safety of Optical Fiber Optic Cable in OPGW; Electric Power Research Institute: Beijing, China, 2014; pp. 30–50. 4. Xi, C.; Qi, L.; Chen, X.; Zhu, H. The Research Report on the Development of the Insulation Parts of OPGW; Electric Power Research Institute: Beijing, China, 2014; pp. 1–5. 5. Wang, X. Analysis of different grounding modes of OPGW for ultra-high voltage transmission. Electr. Eng. Technol. 2011, 11, 47–49. 6. Wang, X. Analysis of different grounding modes of EHV transmission line OPGW. Electr. Technol. 2012, 22, 12–19. 7. Yuan, Z.; Li, W.; Deng, Y. Grounding survey and improvement measures of composite optical ﬁber overhead ground wire for transmission line. Electr. Technol. 2015, 10, 88–91. 8. Pu, H.; Tian, Q.; Huang, S. Discussion on shunt of OPGW. Guizhou Electr. Power Technol. 2013, 16, 61–62. 9. Zhang, Y. Study on Grounding Short Circuit and OPGW Current Calculation of Complex Structure Transmission Line; Zhengzhou University: Henan, China, 2012; pp. 48–55. 10. Li, G. Overhead Transmission Line Single-Phase Grounding Current in the Distribution of OPGW along the Line; North China Electric Power University: Beijing, China, 2015; pp. 4–26. 11. Rao, Z. Research of Fault Current Calculation in Transmission Line with OPGWs Based on Phase Component Method; North China Electric Power University: Beijing, China, 2016; pp. 6–13. 12. Xi, C.; Qi, L.; Chen, X.; Yin, Y. Report on the Safety Demonstration Project of OPGW Feeder Cable; Electric Power Research Institute: Beijing, China, 2014; pp. 6–8. 13. Xiang, J.; Chen, Y.; Wan, X. Application of composite impedance in reducing power loss of OPGW. Shanxi Electr. Power 2016, 44, 82–84. 14. Peng, X.; Mao, X.; Hu, W.; Wang, Y.; Wang, J. Energy-saving grounding technology for overhead ground wire of power transmission line. Power Construct. 2014, 35, 84–90. 15. Cai, L.; Zhou, Z. Discussion on lightning protection of lower part of portal frame of OPGW optical cable substation. Hunan Electr. Power 2012, 32, 43–44. 16. Hu, L. Analysis of lightning protection grounding technology for OPGW cable in power system. Inf. Commun. 2016, 3, 238–239. 17. Qi, L.; Yang, B.; Fan, M. Research on the safety of substation OPGW leading down from the portal frame. Power Syst. Clean Energy 2016, 32, 27–30. Energies 2018, 11, 2441 11 of 11

18. Optical Fiber Composite Overhead Ground Wire (OPGW) Lightning Protection Grounding Technology Guide: DL/T 1378—2014; National Energy Administration: Beijing, China, 2014. 19. Ge, D.; He, H. Study on Lightning Protection Calculation of uhv ac Transmission Line in North Zhejiang-Fuzhou; Electric Power Research Institute: Beijing, China, 2013; pp. 8–22. 20. Chen, C. Analysis of grounding and ﬁxing of OPGW optical cable station. Jiangxi Electr. Power 2015, 7, 53–54. 21. Chen, X.; Xia, C.; Zhu, H. Electrical requirements for ground wire insulator and parallel discharge clearance during insulation reconstruction for OPGW DC de-icing. High Volt. Eng. 2017, 43, 1–6. 22. Guide for Lightning Protection of Overhead Transmission Lines: Q/GDW 11452-2015; State Grid Corporation of China: Beijing, China, 2015. 23. Guide for Grounding Technical of Overhead Transmission Lines: DL/T 1519-2016; National Energy Administration: Beijing, China, 2016.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).